Light-Emitting Device and Electronic Device

ABSTRACT

A highly reliable light-emitting device is provided. Damage to an element due to externally applied physical power is suppressed. Alternatively, in a process of pressure-bonding of an FPC, damage to a resin and a wiring which are in contact with a flexible substrate due to heat is suppressed. A neutral plane at which stress-strain is not generated when a flexible light-emitting device including an organic EL element is deformed, is positioned in the vicinity of a transistor and the organic EL element. Alternatively, the hardness of the outermost surface of a light-emitting device is high. Alternatively, a substrate having a coefficient of thermal expansion of 10 ppm/K or lower is used as a substrate that overlaps with a terminal portion connected to an FPC.

This application is a continuation of copending U.S. application Ser.No. 16/536,609, filed on Aug. 9, 2019 which is a continuation of U.S.application Ser. No. 16/006,281, filed on Jun. 12, 2018 (now U.S. Pat.No. 10,381,599 issued Aug. 13, 2019) which is a continuation of U.S.application Ser. No. 15/484,451, filed on Apr. 11, 2017 (now U.S. Pat.No. 10,003,047 issued Jun. 19, 2018) which is a continuation of U.S.application Ser. No. 14/802,642, filed on Jul. 17, 2015 (now U.S. Pat.No. 9,627,648 issued Apr. 18, 2017) which is a divisional of U.S.application Ser. No. 13/886,474, filed on May 3, 2013 (now U.S. Pat. No.9,088,006 issued Jul. 21, 2015), which are all incorporated herein byreference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a light-emitting device, an electronicdevice, and a lighting device each using organic electroluminescence(hereinafter also referred to as EL).

2. Description of the Related Art

In recent years, research and development have been actively conductedon light-emitting elements utilizing organic EL (organic EL elements).In the fundamental structure of the organic EL element, a layercontaining a light-emitting organic compound (EL layer) is sandwichedbetween a pair of electrodes.

Since the organic EL element is a self-luminous type, a light-emittingdevice including the organic EL element has advantages of highvisibility, no necessity of a backlight, low power consumption, and thelike. In addition, the light-emitting device including the organic ELelement also has advantages that it can be thin and lightweight and itis highly responsive to input signals.

Furthermore, the light-emitting device including an organic EL elementcan achieve reduction in thickness and weight, and further haveflexibility and high impact resistance; therefore, use of such alight-emitting element for a substrate having flexibility (a flexiblesubstrate) has been proposed. The light-emitting element is applied notonly to a light-emitting device but also to a semiconductor device orthe like which operates by utilizing semiconductor characteristics.

For example, Patent Document 1 discloses a flexible active matrixlight-emitting device in which an organic EL element or a transistorserving as a switching element is provided over a film substrate.

In addition, Patent Document 2 discloses an organic EL display deviceincluding, over a resin substrate formed of a flexible film, an organicEL display panel including an organic EL element, an inorganic barrierfilm covering a surface of the resin substrate, and a resin sealing filmsurrounding the organic EL display panel.

As a method for manufacturing a semiconductor device using a flexiblesubstrate, a technique in which a semiconductor element such as a thinfilm transistor is formed over a base material such as a glass substrateor a quartz substrate, and then the semiconductor element is transferredfrom the base material to another base material (for example, a flexiblebase material) has been developed. In order to transfer thesemiconductor element to another base material, a step for separatingthe semiconductor element from the base material that is used forforming the semiconductor element is necessary.

For example, in Patent Document 3, the following peeling technique usinglaser ablation is described. First, a separation layer formed ofamorphous silicon is provided over a substrate, a layer to be peeledwhich includes a thin film element is formed over the separation layer,and the layer to be peeled is bonded to an object to which the layer tobe peeled is transferred, by an adhesive layer. The separation layer isablated by laser irradiation, so that separation is generated in theseparation layer.

Furthermore, in Patent Document 1, a technique is described in whichpeeling is performed by physical force such as human hands. In PatentDocument 1, a metal layer is formed between a substrate and an oxidelayer and separation is generated at an interface between the oxidelayer and the metal layer by utilizing weak bonding between the oxidelayer and the metal layer at the interface, so that the layer to beseparated and the substrate are separated from each other.

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2003-174153 [Patent Document 2] InternationalPublication WO 2006/046679

[Patent Document 3] Japanese Published Patent Application No. H10-125931

SUMMARY OF THE INVENTION

In a light-emitting device including a flexible substrate, however,stress is applied to a transistor and an organic EL element whenphysical force, such as bending or curving, is externally applied, andthus these elements are damaged in some cases.

Damage of any one of a transistor and an organic EL element causes adisplay defect of a light-emitting device.

In view of the above, an object of one embodiment of the presentinvention is to provide a highly reliable light-emitting device in whichdamage caused to a transistor and an organic EL element when physicalforce is externally applied is avoided.

With the structure described in Patent Document 2, there is apossibility that an organic EL element is damaged when mechanicalpressure is locally applied from the surface of the resin sealing film.

Moreover, a resin sealing film is formed thick in order to increasemechanical strength according to Patent Document 2, the followingproblems occur. First, when a resin sealing film is formed thick inorder to increase mechanical strength sufficiently, a device itself isthick and loses its flexibility. Second, when the resin sealing film ismade thick, the device itself rolls up due to stress caused bycontraction of the resin sealing film.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting device having both highflexibility and high mechanical strength.

Moreover, as to mounting of the light-emitting device formed over theflexible substrate, a connector (flexible printed circuit: FPC) or thelike is bonded to a terminal electrode through an anisotropic conductivefilm by thermocompression bonding, so that electrical connection ismade. In this thermocompression bonding step, the flexible substrateitself or a resin and a wiring provided over the substrate may bethermally damaged, which causes display defects of the light-emittingdevice in some cases. This is because the flexible substrate hasflexibility and thus is deformed due to heat, so that a crack isgenerated in the resin and the wiring.

Accordingly, an object of one embodiment of the present invention is toprovide a light-emitting device, in which FPC is pressure-bonded so thatelectrical conduction with a terminal electrode is made without a crackin a wiring.

A light-emitting device of one embodiment of the present inventionincludes, between a pair of flexible substrates, a planarization layer,a transistor over one surface of the planarization layer, an organic ELelement in contact with the other surface of the planarization layer,and an adhesive layer for bonding the flexible substrates. In thelight-emitting device of one embodiment of the present invention, eachlayer is provided so that a difference between the thickness that is adistance from one surface of the light-emitting device to the othersurface of the planarization layer (i.e., a thickness on the sideincluding a transistor) and the thickness that is a distance from theother surface of the planarization layer to the other surface of thelight-emitting device is small; thus, a difference between stressapplied to the transistor and stress applied to the organic EL elementwhen physical power is externally applied is small. Note that in thisspecification, a structure in which A and B are bonded with C includesnot only a structure in which A and B are each in contact with C butalso a structure D positioned over A and E positioned over B are each incontact with C, so that A and B are bonded.

In the light-emitting device, a neutral plane (a plane which does notexpand or contract) in which distortion of stress, such as compressivestress or tensile stress, due to deformation such as bending is notcaused is positioned near both the transistor and the organic ELelement; for example, the neutral plane is positioned in theplanarization layer, the transistor, or the organic EL element. Thus,values of stresses applied to the transistor and the organic EL elementcan be small. Therefore, damage to the transistor and/or the organic ELelement due to bending or curving can be suppressed, thereby achieving ahighly reliable light-emitting device.

In this specification and the like, one surface and the other surface ofa substrate or a planarization layer refer to surfaces which face toeach other.

Specifically, a light-emitting device of one embodiment of the presentinvention includes a planarization layer between one surface of a firstflexible substrate and one surface of a second flexible substrate; atransistor over one surface of the planarization layer; a light-emittingelement including a first electrode which is in contact with the othersurface of the planarization layer and electrically connected to thetransistor, a layer containing a light-emitting organic compound overthe first electrode, and a second electrode over the layer containing alight-emitting organic compound; and an adhesive layer for bonding thefirst flexible substrate and the second flexible substrate. In thelight-emitting device, thickness A that is a distance from the othersurface of the first flexible substrate to the other surface of theplanarization layer is 0.8 to 1.2 times as large as thickness B that isa distance from the other surface of the planarization layer to theother surface of the second flexible substrate. That is, thickness Athat is a distance from the other surface of the first flexiblesubstrate to an interface between the planarization layer and theadhesive layer is 0.8 to 1.2 times as large as thickness B that is adistance from the interface between the planarization layer and theadhesive layer to the other surface of the second flexible substrate.

In the above light-emitting device, it is more preferable that thicknessA be 0.9 to 1.1 times as large as thickness B.

A light-emitting device of one embodiment of the present inventionincludes a planarization layer between one surface of a first flexiblesubstrate and one surface of a second flexible substrate; a transistorover one surface of the planarization layer; a light-emitting elementincluding a first electrode which is in contact with the other surfaceof the planarization layer and electrically connected to the transistor,a layer containing a light-emitting organic compound over the firstelectrode, and a second electrode over the layer containing alight-emitting organic compound; and an adhesive layer for bonding thefirst flexible substrate and the second flexible substrate. In thelight-emitting device, thickness B that is a distance from the othersurface of the planarization layer to the other surface of the secondflexible substrate is 0.8 to 1.2 times as large as thickness A that is adistance from the other surface of the first flexible substrate to theother surface of the planarization layer.

In the above light-emitting device, it is more preferable that thicknessB be 0.9 to 1.1 times as large as thickness A.

An element with low resistance to stress is particularly preferablyprovided near the neutral plane. In this manner, stress applied to theelement when physical power is externally applied can be reduced. Thus,in the above light-emitting device, the planarization layer ispreferably in contact with the transistor.

The light-emitting device of one embodiment of the present invention hasflexibility, is provided over an insulating surface, and includes alight-emitting element. The light-emitting device includes, a layercontaining a light-emitting organic compound is interposed between apair of electrodes, a first protection layer covers the light-emittingelement, and a second protection layer is provided over the firstprotection layer. In addition, it is preferable that the thickness ofthe first protection layer be greater than or equal to 0.1 μm and lessthan 100 μm, and at least a surface of the second protection layer,which does not face the light-emitting element, have a higher hardnessthan a surface of the first protection layer.

Here, since the surface of the first protection layer is softer than thesurface of the second protection layer, in the case where mechanicalpressure is locally applied from the second protection layer side or thelight-emitting device is bent, stress applied to the light-emittingelement can be reduced (such an effect is also referred to as cushioneffect). Note that when the thickness of the first protection layer isless than 0.1 μm, there is a possibility that the light-emitting elementis damaged because the stress cannot be sufficiently reduced. On theother hand, when the thickness of the first protection layer is greaterthan or equal to 100 μm, there is a possibility that the light-emittingdevice loses its flexibility or the light-emitting device winds up.

In addition, the thickness of the first protection layer in a regionoverlapping with an element layer (including the light-emitting elementand the like) may differ from the thickness of the first protectionlayer in a region where the element layer is not provided. Similarly, inthe case where the top surface of the element layer is not flat, thethickness of the first protection layer is not constant. Therefore, thesmallest thickness of the first protection layer is within the abovethickness range.

Thus, the first protection layer is formed to be sufficiently thin,stress applied to the light-emitting element can be reduced and thelight-emitting device can have high flexibility. With the secondprotection layer which is provided on the outermost surface of thelight-emitting device and has a high hardness at least at the surface,damage to an element by mechanical pressure externally applied can besuppressed even in the case where the first protection layer is formedto be thin, whereby the light-emitting device can have high flexibilityand high mechanical strength.

A flexible light-emitting device of one embodiment of the presentinvention includes an element layer comprising a light-emitting element,the light-emitting element comprising a layer containing alight-emitting organic compound between a pair of electrodes; a firstprotection layer and a third protection layer with the light-emittingelement is therebetween; a second protection layer over a surface of thefirst protection layer, which does not face the light-emitting element;and a fourth protection layer over a surface of the third protectionlayer, which does not face the light-emitting element. In thelight-emitting device, the thickness of each of the first protectionlayer and the third protection layer is greater than or equal to 0.1 μmand less than 100 μm, the second protection layer has a higher hardnessat least at a surface that does not face the light-emitting element thana surface of the first protection layer, and the fourth protection layerhas a higher hardness at least at a surface that does not face thelight-emitting element than a surface of the third protection layer.

With the above structure, the light-emitting element (an element layerincluding the light-emitting element) can be protected from both sides,thereby achieving a light-emitting device whose mechanical strength isfurther improved.

At this time, it is preferable that the first protection layer and thethird protection layer be formed with the same material and the secondprotection layer and the fourth protection layer be formed with the samematerial, in which case common materials can be used.

In any of the above light-emitting devices, it is preferable that thefirst protection layer and the second protection layer have a propertyof transmitting visible light, and light from the light-emitting elementbe emitted through the first protection layer and the second protectionlayer.

Particularly in a light-emitting device with a structure in which lightis emitted to the side on which a light-emitting element is formed(i.e., a top emission structure), the light-emitting element is easilydamaged when the mechanical strength of the light-emitting surface islow. Accordingly, with the above structure of the light-emitting device,a highly reliable light-emitting device with a top emission structure,which has high flexibility and high mechanical strength, can be achieved

The light-emitting device with any of the above structures preferablyincludes a color filter overlapping with the light-emitting elementbetween the light-emitting element and the second protection layer. Inthe light-emitting device, the first protection layer and the secondprotection layer preferably have a property of transmitting visiblelight, the light-emitting element preferably emits white light, andlight from the light-emitting element is preferably emitted through thefirst protection layer, the color filter, and the second protectionlayer.

With such a structure, the second protection layer functions not only asa protection layer of the light-emitting element but also as aprotection layer of the color filter, which can prevent the occurrenceof light leakage due to breakage of the color filter.

The light-emitting device with any of the above structures preferablyincludes a fifth protection layer containing a material that exhibitsphotocatalytic activity, which is in contact with the surface of thesecond protection layer.

The contamination with an organic substance on the surface of thelight-emitting device can be suppressed by strong oxidation effect ofthe material that exhibits photocatalytic activity. Such a materialshows a super hydrophilic effect; thus, oil contamination can beprevented from remaining; and contamination can be easily removed byexposing the surface to running water. As a result, the number of timesmechanical pressure is applied to the light-emitting device, forexample, by wiping off an organic substance attached or fixed on thesurface of the light-emitting device, can be reduced. Thus, a defectsuch as a crack formed on the surface of the light-emitting device ordamage to the light-emitting element can be suppressed.

One embodiment of the present invention is a light-emitting deviceincluding a first substrate, a second substrate facing the firstsubstrate, a third substrate partly overlapping with the firstsubstrate, a terminal portion over the third substrate, the terminalportion comprising at least two terminals, a light-emitting elementelectrically connected to the terminal in the terminal portion, and anadhesive layer for bonding the first substrate and the second substrateto seal the light-emitting element between the first substrate and thesecond substrate. In the light-emitting device, the coefficient ofthermal expansion of the third substrate is 10 ppm/K or lower.

By heat applied to an anisotropic conductive film when an FPC ispressure-bonded to the terminal portion, a crack is likely to begenerated in a substrate itself including the terminal portion or aresin and a wiring which are in contact with the substrate. In thelight-emitting device of one embodiment of the present invention, thecoefficient of thermal expansion of the third substrate is as small as10 ppm/K or lower, and thus the third substrate is less likely to bedeformed by heat, so that a crack is less likely to be generated in thethird substrate itself or the resin and the wiring which are in contactwith the third substrate.

Furthermore, since a substrate that does not transmit visible light canbe used as the third substrate, a structure in which the third substratedoes not overlap with the light-emitting element is preferred. With thisstructure, at least one of the first substrate and the second substrateis a substrate having a property of transmitting visible light; thus, apanel with any of a bottom emission structure, a top emission structure,and a dual emission structure can be obtained easily.

It is preferable that the first substrate and the third substrate bebonded to overlap with the terminal portion, and an absolute value(|C−D|) of a difference between a coefficient of thermal expansion C ofa stacked substrate formed by bonding the first substrate and the thirdsubstrate and a coefficient of thermal expansion D of the secondsubstrate be within 10% of the coefficient of thermal expansion C orwithin 10% of the coefficient of thermal expansion D.

When a difference in coefficient of thermal expansion between the secondsubstrate and a stacked substrate of the first substrate and the thirdsubstrate is within 10% of the coefficient of thermal expansion of thestacked substrate or the second substrate, a warp of the substrate inone direction hardly occurs.

Another embodiment of the present invention is a light-emitting deviceincluding a first substrate, a second substrate facing the firstsubstrate, a terminal portion comprising at least two terminals, alight-emitting element electrically connected to the terminal in theterminal portion, and an adhesive layer for bonding the first substrateand the second substrate to seal the light-emitting element between thefirst substrate and the second substrate. In the light-emitting device,the terminal portion is provided in a region where the first substratedoes not overlap with the second substrate, and the coefficient ofthermal expansion of the first substrate is 10 ppm/K or lower.

In the light-emitting device of one embodiment of the present invention,the coefficient of thermal expansion of the first substrate is as smallas 10 ppm/K or lower, and thus the first substrate is less likely to bedeformed by heat, so that a crack is less likely to be generated in thefirst substrate itself or the resin and the wiring which are in contactwith the first substrate.

It is preferable that an absolute value (|E−D|) of a difference betweena coefficient of thermal expansion E of the first substrate and acoefficient of thermal expansion D of the second substrate be within 10%of the coefficient of thermal expansion D or within 10% of thecoefficient of thermal expansion E. When a difference in coefficient ofthermal expansion between the first substrate and the second substrateis within 10% of at least one of the thermal expansion coefficients ofthe first substrate and the second substrate, a warp of the substrate inone direction hardly occurs.

Furthermore, a light-emitting device in which an FPC is attached to theterminal portion by thermocompression bonding is preferable.

Another embodiment of the present invention is an electronic deviceincluding the light-emitting device in a display portion. By theapplication of the above light-emitting device, a highly reliableelectronic device can be achieved.

Note that a light-emitting device in this specification refers to animage display device or a light source (including a lighting device). Inaddition, the light-emitting device includes any of the followingmodules in its category: a module in which a connector such as aflexible printed circuit (FPC) or a tape carrier package (TCP) isattached to a light-emitting device; a module having a TCP provided witha printed wiring board at the end thereof; and a module having anintegrated circuit (IC) directly mounted over a substrate over which alight-emitting element is formed by a chip on glass (COG) method.

In a light-emitting device of one embodiment of the present invention, adifference between stress applied to a transistor and stress applied toan organic EL element when physical power is externally applied issmall, and the values of the stresses are also small. Therefore, damageto the transistor or the organic EL element due to bending or curvingcan be suppressed, thereby achieving a highly reliable light-emittingdevice.

One embodiment of the present invention can provide a light-emittingdevice having both high flexibility and high mechanical strength.

One embodiment of the present invention can provide a light-emittingdevice in which generation of a crack is suppressed in a flexiblesubstrate itself or a resin and a wiring which are in contact with theflexible substrate in the vicinity of an FPC terminal electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a light-emitting device of one embodiment of thepresent invention.

FIGS. 2A and 2B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 3A and 3B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 4A and 4B illustrate a light-emitting device of one embodiment ofthe present invention.

FIGS. 5A to 5E illustrate a method for manufacturing a light-emittingdevice of one embodiment of the present invention.

FIGS. 6A to 6E illustrate a method for manufacturing a light-emittingdevice of one embodiment of the present invention.

FIG. 7 illustrates a light-emitting device of one embodiment of thepresent invention.

FIG. 8 illustrates a light-emitting device of one embodiment of thepresent invention.

FIG. 9 illustrates a light-emitting device of one embodiment of thepresent invention.

FIGS. 10A and 10B illustrate a display device of one embodiment of thepresent invention.

FIG. 11 illustrates a display device of one embodiment of the presentinvention.

FIGS. 12A and 12B illustrate a lighting device of one embodiment of thepresent invention.

FIG. 13 illustrates a light-emitting device of one embodiment of thepresent invention.

FIGS. 14A and 14B illustrate a light-emitting device of one embodimentof the present invention.

FIG. 15 illustrates a light-emitting device of one embodiment of thepresent invention.

FIGS. 16A to 16D illustrate a method for manufacturing a light-emittingdevice of one embodiment of the present invention.

FIGS. 17A to 17F illustrate organic EL elements.

FIGS. 18A to 18E illustrate electronic devices and lighting devices.

FIGS. 19A and 19B illustrate electronic devices.

FIG. 20 shows the results of Example.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments will be described in detail with reference to the drawings.Note that the present invention is not limited to the followingdescription, and it will be easily understood by those skilled in theart that modes and details can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention should not be construed as being limited to thedescription in the following embodiments. Note that in the structures ofthe invention described below, the same portions or portions havingsimilar functions are denoted by the same reference numerals indifferent drawings, and description of such portions is not repeated.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Embodiment 1

In this embodiment, light-emitting devices of embodiments of the presentinvention will be described with reference to FIG. 1 and FIGS. 2A and2B.

FIG. 1 is a schematic view of a light-emitting device of one embodimentof the present invention. The light-emitting device illustrated in FIG.1 includes, between one surface of a first flexible substrate 101 andone surface of a second flexible substrate 111, a planarization layer105, a transistor 103 provided on the one surface side of theplanarization layer 105, an organic EL element 107 provided on the othersurface side of the planarization layer 105, and an adhesive layer 109for bonding the first flexible substrate 101 and the second flexiblesubstrate 111 with the transistor 103, the planarization layer 105, andthe organic EL element 107 are provided therebetween. The organic ELelement 107 is provided on and in contact with the other surface of theplanarization layer 105, and includes a first electrode 171 electricallyconnected to the transistor 103, a layer containing a light-emittingorganic compound (EL layer) 173 over the first electrode 171, and asecond electrode 175 over the EL layer 173.

In the light-emitting device illustrated in FIG. 1, the distance fromthe other surface of the first flexible substrate 101 to the othersurface of the planarization layer 105 is denoted by thickness A. In thelight-emitting device illustrated in FIG. 1, the distance from the othersurface of the planarization layer 105 to the other surface of thesecond flexible substrate 111 is denoted by thickness B. That is,thickness A represents a distance from the other surface of the firstflexible substrate 101 to an interface between the planarization layer105 and the adhesive layer 109 and thickness B represents a distanceform the interface between the planarization layer 105 and the adhesivelayer 109 to the other surface of the second flexible substrate 111.

In the light-emitting device of one embodiment of the present invention,thickness A is 0.8 to 1.2 times (preferably 0.9 to 1.1 times) as largeas thickness B, or thickness B is 0.8 to 1.2 times (preferably 0.9 to1.1 times) as large as thickness A.

In the light-emitting device of one embodiment of the present invention,each layer is provided such that a difference between thickness A (thethickness on the side including the transistor 103) and thickness B (thethickness on the side including the organic EL element 107) is small;therefore, when physical power is externally applied, a differencebetween stress applied to the transistor 103 and stress applied to theorganic EL element 107 is small.

In the light-emitting device, a neutral plane (a plane which does notexpand or contract) in which distortion of stress, such as compressivestress or tensile stress, due to deformation such as bending is notcaused is positioned near both the transistor 103 and the organic ELelement 107; for example, the neutral plane is positioned in theplanarization layer 105, the transistor 103, or the organic EL element107. Thus, values of stresses applied to the transistor 103 and theorganic EL element 107 can be small. Therefore, damage to the transistor103 and/or the organic EL element 107 due to bending or curving can besuppressed, thereby achieving a highly reliable light-emitting device.

As the position of the element is further from the neutral plane, acompressive stress or tensile stress applied to the element becomeslarger. An element with low resistance to stress is particularlypreferably provided near the neutral plane. In this manner, stressapplied to the element when physical power is externally applied can bereduced. Thus, in the light-emitting device of one embodiment of thepresent invention, the planarization layer 105 is preferably in contactwith the transistor 103.

Examples of materials that can be used for the light-emitting device ofone embodiment of the present invention are described. Materials forcomponents of a light-emitting device in any of the followingembodiments can be applied to the components of the light-emittingdevice of this embodiment.

[Flexible Substrate]

A flexible material is used for the first flexible substrate 101 and thesecond flexible substrate 111. In particular, for the substrate throughwhich light emitted from the organic EL element 107 is extracted, amaterial that has a property of transmitting visible light (hereinafterreferred to as light-transmitting property) in addition to flexibilityis used.

Examples of such a material having flexibility and a light-transmittingproperty include polyester resins such as polyethylene terephthalate(PET) and polyethylene naphthalate (PEN), a polyacrylonitrile resin, apolyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC)resin, a polyethersulfone (PES) resin, a polyamide resin, a cycloolefinresin, a polystyrene resin, a polyamide imide resin, and a polyvinylchloride resin. In particular, a material whose coefficient of thermalexpansion is low is preferred, and for example, a polyamide imide resin,a polyimide resin, or PET can be suitably used. A substrate in which afibrous body is impregnated with a resin (also referred to as prepreg)or a substrate whose coefficient of thermal expansion is reduced bymixing an organic resin with an inorganic filler can also be used.

In the case where a fibrous body is contained in the material havingflexibility and a light-transmitting property, a high-strength fiber ofan organic compound or an inorganic compound is used as the fibrousbody. A high-strength fiber is specifically a fiber with a high tensilemodulus of elasticity or a fiber with a high Young's modulus.

Typical examples of a high-strength fiber include a polyvinyl alcoholbased fiber, a polyester based fiber, a polyamide based fiber, apolyethylene based fiber, an aramid based fiber, a polyparaphenylenebenzobisoxazole fiber, a glass fiber, and a carbon fiber. As a glassfiber, there is a glass fiber using E glass, S glass, D glass, Q glass,or the like. These fibers may be used in a state of a woven fabric or anonwoven fabric, and a structure in which this fibrous body isimpregnated with a resin and the resin is cured may be used as theflexible substrate. When the structure including the fibrous body andthe resin is used as the flexible substrate, reliability against bendingor breaking due to local pressure can be increased, which is preferable.

To improve the light extraction efficiency, the refractive index of thematerial having flexibility and a light-transmitting property ispreferably high. For example, a substrate obtained by dispersing aninorganic filler having a high refractive index into an organic resincan have a higher refractive index than the substrate formed of only theorganic resin. In particular, an inorganic filler having a particlediameter as small as 40 nm or less is preferred, because such a fillercan maintain optical transparency.

Furthermore, since the substrate through which light emission is notextracted does not need to have a light-transmitting property, a metalsubstrate or the like can be used in addition to the above-mentionedsubstrates. To obtain flexibility and bendability, the thickness of ametal substrate is preferably greater than or equal to 10 μm and lessthan or equal to 200 μm, more preferably greater than or equal to 20 μmand less than or equal to 50 μm. Since a metal substrate has a highthermal conductivity, heat generated due to light emission of theorganic EL element can be efficiently released.

There is no particular limitation on a material of the metal substrate,but it is preferable to use, for example, aluminum, copper, nickel, ametal alloy such as an aluminum alloy or stainless steel.

The flexible substrate may have a stacked structure in which a hard coatlayer (such as a silicon nitride layer) by which a surface of alight-emitting device is protected from damage, a layer (such as anaramid resin layer) which can disperse pressure, or the like is stackedover a layer of any of the above-mentioned materials. Furthermore, tosuppress a decrease in the lifetime of the organic EL element due tomoisture and the like, a protection film with low water permeability maybe provided. For example, a film including nitrogen and silicon (e.g., afilm including silicon nitride or oxynitride silicon), or a filmincluding nitrogen and aluminum (e.g., a film including aluminumnitride) may be provided.

[Transistor]

The structure of the transistor used for the light-emitting device ofone embodiment of the present invention is not particularly limited. Forexample, a forward staggered transistor or an inverted staggeredtransistor may be used. Furthermore, the transistor may have a top-gatestructure or a bottom-gate structure. In addition, there is noparticular limitation on a material used for the transistor. Forexample, a transistor in which silicon, germanium, or an oxidesemiconductor is used in a channel formation region can be employed.

[Planarization Layer]

The planarization layer 105 is an insulating film that is provided forreducing surface roughness caused by the transistor 103. For example, anorganic material such as polyimide, acrylic, polyamide, polyimide amide,or a benzocyclobutene-based resin can be used. Alternatively, alow-dielectric constant material (a low-k material) or the like can beused. Furthermore, the planarization layer 105 may be formed by stackingtwo or more insulating films.

[Organic EL Element]

The structure of the organic EL element used for the light-emittingdevice of one embodiment of the present invention is not particularlylimited. The organic EL element may have a top emission structure, abottom emission structure, or a dual emission structure. Examples of astructure of the organic EL element will be described in detail inEmbodiment 7.

[Adhesive Layer]

In the case of using an organic EL element with a top emissionstructure, the adhesive layer 109 is formed with a material thattransmits light from the organic EL element. For example, any of avariety of types of curable adhesive, e.g., a light curable adhesivesuch as a UV curable adhesive, a reactive curable adhesive, a thermalcurable adhesive, and an anaerobic adhesive can be used. Examples ofmaterials of such adhesives include an epoxy resin, an acrylic resin, asilicone resin, a phenol resin, and an imide resin. In particular, amaterial with low moisture permeability, such as an epoxy resin, ispreferred. For example, the above adhesive may include a drying agent(such as zeolite). Accordingly, deterioration of the organic EL elementcan be suppressed. The refractive index of the adhesive layer ispreferably high. For example, by mixing a filler with a high refractiveindex (e.g., titanium oxide or zirconium) into the adhesive layer, theefficiency of light extraction from the organic EL element can beimproved.

Also in the case of using an organic EL element with a bottom emissionstructure, a material similar to the material used in the case of anorganic EL element with a top emission structure can be used for theadhesive layer 109. Note that the adhesive layer 109 does notnecessarily have a light-transmitting property.

Structural Example 1 of Light-Emitting Device

FIGS. 2A and 2B illustrate an example of a specific structure of alight-emitting device to which one embodiment of the present inventionis applied. FIG. 2A is a plan view of a light-emitting device of oneembodiment of the present invention. The light-emitting deviceillustrated in FIG. 2A includes a pixel portion 4502, a signal linedriver circuit 4503 a, a signal line driver circuit 4503 b, a scan linedriver circuit 4504 a, a scan line driver circuit 4504 b, and an FPC4505. FIG. 2B is a cross-sectional view taken along an alternate longand short dash line C-D in FIG. 2A.

The light-emitting device illustrated in FIG. 2B includes, between onesurface of the first flexible substrate 101 and one surface of thesecond flexible substrate 111, the planarization layer 105, thetransistor 103 provided on the one surface side of the planarizationlayer 105, the organic EL element 107 provided on the other surface sideof the planarization layer 105, and the adhesive layer 109 for bondingthe first flexible substrate 101 and the second flexible substrate 111.

A base film 121 is provided over one surface of the first flexiblesubstrate 101, and the transistor 103 is provided over the base film121. The transistor 103 includes a gate electrode 131, a gate insulatingfilm 133, a semiconductor layer 135, and a conductive layer 137 a and aconductive layer 137 b each functions as a source electrode or a drainelectrode.

An insulating film 141 and the planarization layer 105 which cover thetransistor are provided over the transistor 103.

The organic EL element 107 includes the first electrode 171 electricallyconnected to the conductive layer 137 b, the EL layer 173, and thesecond electrode 175. An end portion of the first electrode 171 iscovered with a partition wall 161.

In FIG. 2B, the distance from the other surface of the first flexiblesubstrate 101 to the other surface of the planarization layer 105 isdenoted by thickness A. In FIG. 2B, the distance from the other surfaceof the planarization layer 105 to the other surface of the secondflexible substrate 111 is denoted by thickness B.

In the light-emitting device of one embodiment of the present invention,thickness A is 0.8 to 1.2 times (preferably 0.9 to 1.1 times) as largeas thickness B, or thickness B is 0.8 to 1.2 times (preferably 0.9 to1.1 times) as large as thickness A. Therefore, when physical power isexternally applied, a difference between pressure applied to thetransistor 103 and pressure applied to the organic EL element 107 issmall.

In the light-emitting device, a neutral plane (a plane which does notexpand or contract) in which distortion of stress, such as compressivestress or tensile stress, due to deformation such as bending is notcaused is positioned near both the transistor 103 and the organic ELelement 107; for example, the neutral plane is positioned in theplanarization layer 105, the transistor 103, or the organic EL element107. Thus, values of stresses applied to the transistor 103 and theorganic EL element 107 can be small. Therefore, damage to the transistor103 and/or the organic EL element 107 due to bending or curving can besuppressed, thereby achieving a highly reliable light-emitting device.

Examples of materials that can be used for the light-emitting deviceillustrated in FIGS. 2A and 2B are described. Note that the materials ofcomponents described above, such as the flexible substrate, are notrepeatedly described.

[Base Film]

For stable characteristics of the transistor 103, or the like, the basefilm 121 is preferably provided. The base film 121 can be formed with aninorganic insulating film of silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, or the like to have a single-layerstructure or a layered structure. The base film 121 can be formed by asputtering method, a plasma CVD method, a coating method, a printingmethod, or the like. For example, a silicon oxide film may be formed bya sputtering method to a thickness greater than or equal to 10 nm andless than or equal to 3000 nm, preferably greater than or equal to 200nm and less than or equal to 1500 nm. Note that the base film 121 is notnecessarily provided unless needed.

[Transistor] <Gate Electrode>

The gate electrode 131 can be formed to have a single-layer structure ora stacked-layer structure using any of metal materials such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, and scandium, or an alloy material which contains any ofthese elements, for example.

<Gate Insulating Film>

The gate insulating film 133 can be formed to have a single-layerstructure or a stacked-layer structure using any of silicon oxide,silicon nitride, silicon oxynitride, silicon nitride oxide, and aluminumoxide by a plasma CVD method, a sputtering method, or the like. Forexample, a silicon oxynitride film may be formed using a deposition gascontaining SiH₄ and N₂O by a plasma CVD method.

<Semiconductor Layer>

The semiconductor layer 135 can be formed using a silicon semiconductoror an oxide semiconductor. There is no particular limitation on thecrystallinity of a semiconductor, and any of an amorphous semiconductoror a semiconductor having crystallinity (a microcrystallinesemiconductor, a polycrystalline semiconductor, and a semiconductorpartly including crystal regions) may be used. A semiconductor havingcrystallinity is preferably used, in which case deterioration oftransistor characteristics can be suppressed. As a siliconsemiconductor, amorphous silicon, single crystal silicon,polycrystalline silicon, or the like can be used. As an oxidesemiconductor, an In—Ga—Zn—O-based metal oxide or the like can be used.An oxide semiconductor that can be applied to one embodiment of thepresent invention will be described in Embodiment 8.

<Source Electrode and Drain Electrode>

As the conductive layer 137 a and a conductive layer 137 b eachfunctioning as a source electrode or a drain electrode, for example, ametal film containing an element selected from aluminum, chromium,copper, tantalum, titanium, molybdenum, and tungsten, or a metal nitridefilm containing any of the above elements (e.g., a titanium nitridefilm, a molybdenum nitride film, or a tungsten nitride film) can beused. A structure may also be used in which a film of ahigh-melting-point metal such as titanium, molybdenum, or tungsten, or anitride film of any of these metals (a titanium nitride film, amolybdenum nitride film, or a tungsten nitride film) is stacked eitheror both of over and under a metal film of aluminum, copper, or the like.

Alternatively, the conductive layers 137 a and 137 b may be formed witha conductive metal oxide. As the conductive metal oxide, indium oxide(e.g., In₂O₃), tin oxide (e.g., SnO₂), zinc oxide (ZnO), indium tinoxide (ITO), indium zinc oxide (e.g., In₂O₃—ZnO), or any of these metaloxide materials in which silicon oxide is contained can be used.

[Insulating Film]

The insulating film 141 has an effect of suppressing diffusion ofimpurities into a semiconductor included in a transistor. As theinsulating film 141, an inorganic insulating film such as a siliconoxide film, a silicon oxynitride film, or an aluminum oxide film can beused.

[Partition Wall]

For the partition wall 161, an organic insulating material or aninorganic insulating material is used. It is particularly preferablethat the partition wall be formed using a photosensitive resin materialto have an opening such that a sidewall of the opening has an inclinedsurface with continuous curvature.

Furthermore, a sealing film with low moisture permeability may be formedbetween the adhesive layer 109 and the second electrode 175. As thesealing film with low moisture permeability, for example, silicon oxide,silicon nitride, aluminum oxide, or the like can be used.

In a light-emitting device of this embodiment of the present invention,a difference between stress applied to a transistor and stress appliedto an organic EL element when physical power is externally applied issmall, and the values of the stresses are small. Therefore, damage tothe transistor or the organic EL element due to bending or curving canbe suppressed, thereby achieving a highly reliable light-emittingdevice.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 2

In this embodiment, a light-emitting device of one embodiment of thepresent invention will be described with reference to FIGS. 3A and 3B,FIGS. 4A and 4B, FIGS. 5A to 5E, and FIGS. 6A to 6E.

The light-emitting device of this embodiment is manufactured by atechnique in which an element such as a transistor is manufactured overa formation substrate, and then the element is transferred from theformation substrate to a flexible substrate.

Structural Example 2 of Light-Emitting Device

FIGS. 3A and 3B illustrate an example of a specific structure of alight-emitting device to which one embodiment of the present inventionis applied. FIG. 3A is a plan view of a light-emitting device of oneembodiment of the present invention. The light-emitting deviceillustrated in FIG. 3A includes the pixel portion 4502, a signal linedriver circuit 4503, a scan line driver circuit 4504, and the FPC 4505.FIG. 3B is a cross-sectional view taken along an alternate long andshort dash line E-F in FIG. 3A.

The light-emitting device illustrated in FIG. 3B includes, between onesurface of the first flexible substrate 101 and one surface of thesecond flexible substrate 111, the planarization layer 105, thetransistor 103 provided on the one surface side of the planarizationlayer 105, the organic EL element 107 provided on the other surface sideof the planarization layer 105, and the adhesive layer 109 for bondingthe first flexible substrate 101 and the second flexible substrate 111.

The one surface of the first flexible substrate 101 and the base film121 are bonded with an adhesive layer 123. A plurality of transistors(the transistor 103, a transistor 143, a transistor 145, and atransistor 147) is provided over the base film 121.

In addition, the insulating film 141 and the planarization layer 105which cover the plurality of transistors are provided. The planarizationlayer 105 has a stacked structure of a first planarization layer 151 anda second planarization layer 153. A wiring 155 that electricallyconnects the source electrode or drain electrode of the transistor 103and a lower electrode of the organic EL element is provided between thefirst planarization layer 151 and the second planarization layer 153.

FIG. 3B illustrates an organic EL element with a bottom emissionstructure. Over the insulating film 141, a color filter 157 is providedin a region overlapping with an emission region of the organic ELelement 107. An end portion of the lower electrode of the organic ELelement is covered with the partition wall 161.

Structural Example 3 of Light-Emitting Device

FIGS. 4A and 4B illustrate an example of a specific structure of alight-emitting device to which one embodiment of the present inventionis applied. FIG. 4A is a plan view of a light-emitting device of oneembodiment of the present invention. The light-emitting deviceillustrated in FIG. 4A includes the pixel portion 4502, the signal linedriver circuit 4503 a, the signal line driver circuit 4503 b, the scanline driver circuit 4504 a, the scan line driver circuit 4504 b, and theFPC 4505. FIG. 4B is a cross-sectional view taken along an alternatelong and short dash line G-H in FIG. 4A.

The light-emitting device illustrated in FIG. 4B includes, between onesurface of the first flexible substrate 101 and one surface of thesecond flexible substrate 111, the planarization layer 105, thetransistor 103 provided on the one surface side of the planarizationlayer 105, the organic EL element 107 provided on the other surface sideof the planarization layer 105, and the adhesive layer 109 for bondingthe first flexible substrate 101 and the second flexible substrate 111.

The one surface of the first flexible substrate 101 and the base film121 are bonded with the adhesive layer 123. A plurality of transistors(the transistor 103 and the like) is provided over the base film 121.

In addition, the insulating film 141 and the planarization layer 105which cover the plurality of transistors are provided. The partitionwall 161 that covers an end portion of the lower electrode of theorganic EL element 107 is provided over the planarization layer 105.

In the light-emitting device illustrated in FIG. 4B, a spacer thatadjusts a space (also referred to as cell gap) between the firstflexible substrate 101 and the second flexible substrate 111 and anauxiliary wiring that is electrically connected to an upper electrode ofthe organic EL element may be further provided between the partitionwall 161 and the second flexible substrate 111.

One surface of the second flexible substrate 111 and a base film 185 arebonded with an adhesive layer 187. Over the base film 185, a colorfilter 181 provided in a region overlapping with the emission region ofthe organic EL element 107 and a black matrix 183 provided in a regionoverlapping with the partition wall 161 exist.

In FIG. 3B and FIG. 4B, the distance from the other surface of the firstflexible substrate 101 to the other surface of the planarization layer105 is denoted by thickness A, and the distance from the other surfaceof the planarization layer 105 to the other surface of the secondflexible substrate 111 is denoted by thickness B.

In the light-emitting device of one embodiment of the present invention,thickness A is 0.8 to 1.2 times (preferably 0.9 to 1.1 times) as largeas thickness B, or thickness B is 0.8 to 1.2 times (preferably 0.9 to1.1 times) as large as thickness A. Therefore, when physical power isexternally applied, a difference between pressure applied to thetransistor 103 and pressure applied to the organic EL element 107 issmall.

In the light-emitting device, a neutral plane (a plane which does notexpand or contract) in which distortion of stress, such as compressivestress or tensile stress, due to deformation such as bending is notcaused is positioned near both the transistor 103 and the organic ELelement 107; for example, the neutral plane is positioned in theplanarization layer 105, the transistor 103, or the organic EL element107. Thus, values of stresses applied to the transistor 103 and theorganic EL element 107 can be small. Therefore, damage to the transistor103 and/or the organic EL element 107 due to bending or curving can besuppressed, thereby achieving a highly reliable light-emitting device.

Examples of materials that can be used for the light-emitting devicesillustrated in FIGS. 3A and 3B and FIGS. 4A and 4B are described. Notethat the materials of components described above, such as the flexiblesubstrate, are not repeatedly described. The materials for thecomponents of the light-emitting device in any of the other embodimentscan be applied to the components of the light-emitting device of thisembodiment.

[Adhesive Layer]

A material similar to the material of the adhesive layer 109 can beemployed as a material that can be used for the adhesive layer 123 andthe adhesive layer 187. In particular, a material used for the sidethrough which light emitted from the light-emitting element is extractedis preferably a material with a high refractive index.

[Color Filter and Black Matrix]

A color filter is provided in order to control the color of lightemitted from the organic EL element. For example, in the case where awhite light-emitting element is used for a full-color light-emittingdevice, different organic EL elements which overlap with theirrespective color filters are used. In that case, the color filters mayhave three colors of red (R), green (G), and blue (B) or four colorswith yellow (Y) in addition to RGB. Each color filter is formed in adesired position with various materials by a printing method, an inkjetmethod, an etching method using a photolithography technique, or thelike.

A black matrix (also referred to as light-blocking film) is providedbetween adjacent color filters. A black matrix blocks light emitted froman adjacent organic EL element to prevent color mixture between adjacentorganic EL elements. Here, a color filter is provided such that its endportion overlaps with a black matrix, so that light leakage can besuppressed. The black matrix can be formed with a material that blockslight emitted from the organic EL element, and for example, metal or anorganic resin can be used. Note that the black matrix may be provided tooverlap with a region other than a pixel portion, for example, the blackmatrix may be provided in a driver circuit portion.

In the light-emitting device illustrated in FIG. 4B, an overcoatcovering the color filter 181 and the black matrix 183 may be provided.With the overcoat, an impurity and the like contained in the colorfilter can be prevented from dispersing into the organic EL element. Theovercoat is formed with a material that transmits light emitted from theorganic EL element; for example, an inorganic insulating film such as asilicon nitride film or a silicon oxide film, an organic insulating filmsuch as an acrylic film or a polyimide film can be used, and further, astacked structure of an organic insulating film and an inorganicinsulating film may be employed.

In this embodiment, a light-emitting device using a color filter methodis described as an example, but one embodiment of the present inventionis not limited thereto. For example, a separate coloring method or acolor conversion method may be used.

[Base Film]

The base film 185 can be formed with a material similar to the materialfor the base film 121. The base film 185 is not necessarily providedunless needed.

[Wiring]

A material with high conductivity may be used for the wiring 155, andfor example, a material that can be used for an electrode of atransistor can be used.

A spacer over the partition wall 161 can be formed with an inorganicinsulating material, an organic insulating material, or a metalmaterial. As the organic insulating material, for example, a negative orpositive type photosensitive resin material, a non-photosensitive resinmaterial, or the like can be used. As the metal material, titanium,aluminum, or the like can be used.

Methods for manufacturing the light-emitting devices illustrated inFIGS. 3A and 3B and FIGS. 4A and 4B will be described with reference toFIGS. 5A to 5E and 6A to 6E.

<Manufacturing Method 1 of Light-Emitting Device>

A methods for manufacturing the light-emitting device illustrated inFIGS. 3A and 3B will be described with reference to FIGS. 5A to 5E.

First, a separation layer 503 is formed over a formation substrate 501,and a layer 505 to be separated (hereinafter referred to as layer 505)is formed over the separation layer 503 (FIG. 5A).

There is no particular limitation on a layer formed as the layer 505.Here, for the layer 505, a protection layer, the base film 121, thetransistor 103, the insulating film 141, the color filter 157, theplanarization layer 105, the lower electrode of the organic EL element107, and the partition wall 161 are formed in this order. For the layer505, an EL layer of the organic EL element 107, the upper electrode ofthe organic EL element 107, a sealing film for sealing the organic ELelement 107, and the like may be further formed. Alternatively, only theprotection layer and the base film 121 may be formed for the layer 505,and after the separation and transferring, the transistor 103 and thelike may be formed.

The formation substrate 501 may be a glass substrate, a quartzsubstrate, a sapphire substrate, a ceramic substrate, a metal substrate,or the like.

For the glass substrate, for example, a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass can be used. In the case where the temperature of the heattreatment to be performed later is high, a glass substrate whose strainpoint is 730° C. or higher is preferably used. Note that by containing alarge amount of barium oxide (BaO), a glass substrate which isheat-resistant and more practical can be obtained. Alternatively,crystallized glass or the like may be used.

In the case where a glass substrate is used as the formation substrate501, an insulating film such as a silicon oxide film, a siliconoxynitride film, a silicon nitride film, or a silicon nitride oxide filmis preferably formed between the formation substrate 501 and theseparation layer 503, in which case a contaminanation from the glasssubstrate can be prevented.

The separation layer 503 has a single-layer structure or a layeredstructure containing an element selected from tungsten, molybdenum,titanium, tantalum, niobium, nickel, cobalt, zirconium, zinc, ruthenium,rhodium, palladium, osmium, iridium, and silicon; an alloy materialcontaining any of the elements; or a compound material containing any ofthe elements. A crystal structure of a layer containing silicon may beamorphous, microcrystal, or polycrystal. Furthermore, a metal oxide suchas aluminum oxide, gallium oxide, zinc oxide, titanium dioxide, indiumoxide, indium tin oxide, indium zinc oxide, or InGaZnO (IGZO) can beused for the separation layer 503.

The separation layer 503 can be formed by a sputtering method, a plasmaCVD method, a coating method, a printing method, or the like. Note thata coating method includes a spin coating method, a droplet dischargemethod, and a dispensing method.

In the case where the separation layer 503 has a single layer structure,a tungsten layer, a molybdenum layer, or a layer containing a mixture oftungsten and molybdenum is preferably formed. Alternatively, a layercontaining an oxide or an oxynitride of tungsten, a layer containing anoxide or an oxynitride of molybdenum, or a layer containing an oxide oran oxynitride of a mixture of tungsten and molybdenum may be formed.Note that the mixture of tungsten and molybdenum corresponds to an alloyof tungsten and molybdenum, for example.

In the case where the separation layer 503 has a stacked structureincluding a layer containing tungsten and a layer containing an oxide oftungsten, it may be utilized that the layer containing tungsten isformed first and an insulating layer formed of oxide is formed thereoverso that a layer containing an oxide of tungsten is formed at theinterface between the tungsten layer and the insulating layer.Alternatively, the layer containing an oxide of tungsten may be formedby performing thermal oxidation treatment, oxygen plasma treatment,treatment with a highly oxidizing solution such as ozone water, or thelike on the surface of the layer containing tungsten. Plasma treatmentor heat treatment may be performed in an atmosphere of oxygen, nitrogen,nitrous oxide alone, or a mixed gas of any of these gasses and anothergas. Surface condition of the separation layer 503 is changed by theplasma treatment or heat treatment, whereby adhesion between theseparation layer 503 and the protection layer formed later can becontrolled.

The protection layer included in the layer 505 preferably has asingle-layer structure or a layered structure including any of siliconnitride, silicon oxynitride, silicon nitride oxide, and the like.

The protection layer can be formed by a sputtering method, a plasma CVDmethod, a coating method, a printing method, or the like. For example,the protection layer is formed at a temperature higher than or equal to250° C. and lower than or equal to 400° C. by a plasma CVD method,whereby a dense film having very low water permeability can be formed.The thickness of the protection layer is preferably greater than orequal to 10 nm and less than or equal to 3000 nm, further preferablygreater than or equal to 200 nm and less than or equal to 1500 nm. Notethat in the case where the base film 121 also functions as theprotection layer, the protection layer is not necessarily provided.

Next, the layer 505 and a temporary supporting substrate 507 are bondedwith an adhesive 509 for separation, and the layer 505 is separated fromthe formation substrate 501 along the separation layer 503. Accordingly,the layer 505 is placed on the temporary supporting substrate 507 side(FIG. 5B).

The temporary supporting substrate 507 may be a glass substrate, aquartz substrate, a sapphire substrate, a ceramic substrate, a metalsubstrate, or the like. Alternatively, a plastic substrate that canwithstand a processing temperature of this embodiment may be used, or aflexible film-like substrate may be used.

An adhesive with which the temporary supporting substrate 507 and thelayer 505 can be chemically or physically separated when necessary, suchas an adhesive that is soluble in water or a solvent or an adhesivewhich is capable of being plasticized upon irradiation of UV light orthe like, is used as the adhesive 509 for separation.

Any of various methods can be used as appropriate as the process fortransferring the layer to be separated to the temporary supportingsubstrate. For example, in the case where a layer including a metaloxide film is formed as the separation layer on the side in contact withthe layer to be separated, the metal oxide film is embrittled bycrystallization, whereby the layer to be separated can be separated fromthe formation substrate. Alternatively, in the case where an amorphoussilicon film containing hydrogen is formed as the separation layerbetween the formation substrate having high heat resistance and thelayer to be separated, the amorphous silicon film is removed by laserlight irradiation or etching, whereby the layer to be separated can beseparated from the formation substrate. Alternatively, after a layerincluding a metal oxide film is formed as the separation layer on theside in contact with the layer to be separated, the metal oxide film isembrittled by crystallization, and part of the separation layer isremoved by etching using a solution or a fluoride gas such as NF₃, BrF₃,or ClF₃, whereby the separation can be performed at the embrittled metaloxide film. Further alternatively, a method may be used in which a filmcontaining nitrogen, oxygen, hydrogen, or the like (for example, anamorphous silicon film containing hydrogen, an alloy film containinghydrogen, an alloy film containing oxygen, or the like) is used as theseparation layer, and the separation layer is irradiated with laserlight to release the nitrogen, oxygen, or hydrogen contained in theseparation layer as a gas, thereby promoting separation between thelayer to be separated and the formation substrate.

Alternatively, it is possible to use a method in which the formationsubstrate provided with the layer to be separated is removedmechanically or by etching using a solution or a fluoride gas such asNF₃, BrF₃, or ClF₃, or the like. In this case, the separation layer isnot necessarily provided.

When a plurality of the above-described separation methods is combined,the transfer process can be conducted easily. For example, separationcan be performed with physical force (by a machine or the like) afterperforming laser light irradiation, etching on the separation layer witha gas, a solution, or the like, or mechanical removal with a sharpknife, scalpel, or the like so that the separation layer and the layerto be separated can be easily separated from each other.

Alternatively, separation of the layer to be separated from theformation substrate may be carried out after a liquid is made topenetrate an interface between the separation layer and the layer to beseparated. Further alternatively, the separation may be performed whilepouring a liquid such as water during the separation.

As another separation method, in the case where the separation layer 503is formed using tungsten, it is preferable that the separation beperformed while etching the separation layer 503 using a mixed solutionof ammonium water and a hydrogen peroxide solution.

Next, with the adhesive layer 123 which is formed with an adhesivedifferent from the adhesive 509 for separation, the first flexiblesubstrate 101 is bonded to the separation layer 503 exposed by theseparation from the formation substrate 501 or the layer 505 in whichthe protection layer is exposed by the separation from the formationsubstrate 501 (FIG. 5C).

After that, the temporary supporting substrate 507 is removed bydissolving or plasticizing the adhesive 509 for separation. After thetemporary supporting substrate 507 is removed, the adhesive 509 forseparation is removed using water, a solvent, or the like such that thelower electrode of the organic EL element 107 is exposed (FIG. 5D).

Through the above steps, the layer 505 which includes components fromthe transistor 103 to the lower electrode of the organic EL element 107can be formed over the first flexible substrate 101.

After that, the EL layer and the upper electrode (which are notillustrated) of the organic EL element 107 are formed, and then thesecond flexible substrate 111 is bonded thereto with the adhesive layer109 (FIG. 5E).

Finally, the FPC 4505 is attached to each electrode of an input-outputterminal portion with the use of an anisotropic conductive member. An ICchip or the like may be mounted if necessary.

In the above manner, the light-emitting device illustrated in FIGS. 3Aand 3B can be manufactured.

Note that the separation layer is not necessarily provided in the casewhere separation at an interface between the formation substrate and thelayer to be separated is possible. For example, a glass substrate isused as the formation substrate 501, an organic resin film such as apolyimide film is formed in contact with the glass substrate, and theprotection layer, the base film, the transistor, and the like are formedover the organic resin film. In this case, heating the organic resinfilm enables the separation at the interface between the formationsubstrate 501 and the organic resin film. Then, the organic resin filmand the first flexible substrate 101 may be bonded with the adhesivelayer 123. Alternatively, separation at the interface between a metallayer and the organic resin film may be performed in the followingmanner; the metal layer is provided between the formation substrate andthe organic resin film and current is made to flow in the metal layer sothat the metal layer is heated.

<Manufacturing Method 2 of Light-emitting Device>

A methods for manufacturing the light-emitting device illustrated inFIGS. 4A and 4B will be described with reference to FIGS. 6A to 6E.

First, the separation layer 503 is formed over a formation substrate501, and the layer 505 is formed over the separation layer 503 (FIG.6A).

Here, as the layer 505, a protection layer, the base film 121, thetransistor 103, the insulating film 141, the planarization layer 105,the lower electrode of the organic EL element 107, the partition wall161, and the EL layer and the upper electrode of the organic EL element107 are formed in this order. In addition, a sealing film that seals theorganic EL element 107 may be formed.

A separation layer 553 is formed over a formation substrate 551, and alayer 555 to be separated is formed over the separation layer 553 (FIG.6B).

Here, for the layer 555 to be separated, the base film 185, the blackmatrix 183, and the color filter 181 are formed in this order. Aprotection layer may be formed between the separation layer 553 and thebase film 185.

Next, the formation substrate 501 and the formation substrate 551 arebonded with the adhesive layer 109 (FIG. 6C).

Then, the layer 505 is separated from the formation substrate 501 alongthe separation layer 503. Next, with the adhesive layer 123, the firstflexible substrate 101 is bonded to the layer 505 with the separationlayer 503 or the protection layer (or the base film 121) exposed by theseparation from the formation substrate 501 (FIG. 6D).

Similarly, the layer 555 is separated from the formation substrate 551along the separation layer 553. Next, with the adhesive layer 187, thesecond flexible substrate 111 is bonded to the layer 555 with theseparation layer 553 or the base film 185 exposed by the separation fromthe formation substrate 551 (FIG. 6E).

Finally, the FPC 4505 is attached to each electrode of an input-outputterminal portion with use of an anisotropic conductive member. An ICchip or the like may be mounted if necessary.

In the above manner, the light-emitting device illustrated in FIGS. 4Aand 4B can be manufactured.

This embodiment can be implemented in appropriate combination with anyof the structures described in the other embodiments.

Embodiment 3

In this embodiment, a light-emitting device of one embodiment of thepresent invention will be described with reference to FIG. 7, FIG. 8,and FIG. 9.

Structural Example 4 of Light-Emitting Device

FIG. 7 is a schematic cross-sectional view of a light-emitting device400 described as an example in this embodiment.

The light-emitting device 400 includes an element layer 420 including alight-emitting element 410 provided over the substrate 401, a firstprotection layer 411 covering the element layer 420, and a secondprotection layer 412 provided over the first protection layer 411.

The substrate 401 has flexibility. The light-emitting element 410 isformed over an insulating surface of the substrate 401.

The light-emitting element 410 is a stack in which a first electrode403, an EL layer 405, and a second electrode 407 are stacked in thisorder over the substrate 401. The EL layer 405 includes at least alight-emitting organic compound. At least one of the first electrode 403and the second electrode 407 has a light-transmitting property withrespect to light emitted from the EL layer 405. The light-emittingelement 410 can emit light by voltage application between the firstelectrode 403 and the second electrode 407 with the EL layer 405provided therebetween.

Here, the element layer 420 is a layer including the light-emittingelement 410. Note that the element layer 420 includes at least thelight-emitting element 410 and may include a component other than thelight-emitting element 410 (e.g., a transistor or a wiring), asdescribed in the following example.

The first protection layer 411 is a layer for protecting the elementlayer 420 and is provided to cover the element layer 420. The thicknessof the first protection layer 411 is greater than or equal to 0.1 μm andless than 100 μm.

Here, the thickness of the first protection layer 411 in a regionoverlapping with the element layer 420 may differ from the thickness ofthe first protection layer 411 in a region where the element layer 420is not provided. Similarly, in the case where the top surface of theelement layer 420 is not flat, the thickness of the first protectionlayer 411 is not constant. Therefore, the smallest thickness of thefirst protection layer 411 is within the above thickness range.

The second protection layer 412 is provided over the first protectionlayer 411. The second protection layer 412 has a higher hardness atleast at the surface that does not face the element layer 420 than asurface of the first protection layer 411.

For example, in Pencil Test (pencil hardness test) standardized byISO15184, the surface of the second protection layer 412, which does notface the element layer 420, has a hardness of H or harder, preferably 2Hor harder, more preferably 3H or harder.

Note that in the case where the pencil hardness test is employed as amethod for measuring the hardness, when the hardness of the surface ofthe second protection layer 412, which does not face the element layer420, is determined to be the same level as the first protection layer411, other hardness indexes (e.g., Vickers hardness or Knoop hardness)can be used for comparison.

Here, since the surface of the first protection layer 411 is softer thanthe surface of the second protection layer 412, which does not face theelement layer 420, in the case where mechanical pressure is locallyapplied from the second protection layer 412 side or the light-emittingdevice 400 is bent, stress applied to the element layer 420 can bereduced. Here, when the thickness of the first protection layer 411 isless than 0.1 μm, there is a possibility that the light-emitting element410 (or a transistor, a wiring, or the like) included in the elementlayer 420 is damaged because the stress cannot be sufficiently reduced.On the other hand, when the thickness of the first protection layer 411is greater than or equal to 100 μm, there is a possibility that thelight-emitting device 400 loses its flexibility or the light-emittingdevice 400 winds up.

Thus, the first protection layer 411 is formed to be sufficiently thin,stress applied to the element layer 420 can be reduced and thelight-emitting device 400 can have high flexibility. With the secondprotection layer 412 which is provided on the outermost surface of thelight-emitting device and has a high hardness at least at the surfacethat does not face the element layer 420, damage to an element includedin the element layer 420, such as the light-emitting element 410, bymechanical pressure externally applied can be suppressed even in thecase where the first protection layer 411 is formed to be thin, wherebythe light-emitting device 400 can have high flexibility and highmechanical strength.

Note that another protection layer may be provided between the firstprotection layer 411 and the second protection layer 412. For example,an organic resin is used for the first protection layer 411, a thin filmor the like is provided over the first protection layer 411, and aninorganic insulating film is formed as the second protection layer 412over a surface of the thin film that does no face the element layer 420.With the other protection layer between the first protection layer 411and the second protection layer 412, mechanical strength of thelight-emitting device 400 can be further improved.

Structural Example 5 of Light-Emitting Device

Furthermore, a third protection layer and a fourth protection layer maybe provided below the element layer 420 including the light-emittingelement 410 instead of the substrate 401.

A difference between the light-emitting device 400 described inStructural Example 4 and a light-emitting device 450 illustrated in FIG.8 is the structure of a layer under the element layer 420.

In the light-emitting device 450, the element layer 420 is provided overan insulating layer 409, a third protection layer 413 is provided belowthe element layer 420 with the insulating layer 409 providedtherebetween, and a fourth protection layer 414 is provided under thethird protection layer 413.

The thickness of the third protection layer 413 is greater than or equalto 0.1 and less than 100 μm. The hardness of the fourth protection layer414 has a higher hardness at least at the lower surface (i.e., thesurface that does not face the element layer 420) than the thirdprotection layer 413.

For example, the lower surface of the fourth protection layer 414 has ahardness of H or harder, preferably 2H or harder, more preferably 3H orharder.

With the above-described structure where the element layer 420 issandwiched between a stack of the first protection layer 411 and thesecond protection layer 412 and a stack of the third protection layer413 and the fourth protection layer 414, damage to components of theelement layer 420 due to pressure from any plane of the light-emittingdevice 450 can be suppressed, whereby a highly reliable light-emittingdevice whose mechanical strength is improved can be achieved.

Here, the third protection layer 413 is preferably formed with the samematerial as the first protection layer 411, in which case a commonmaterial can be used at the formation. Similarly, the fourth protectionlayer 414 is preferably formed with the same material as the secondprotection layer 412.

Note that the thickness of the first protection layer 411 may differfrom that of the third protection layer 413. Specifically, since thethickness of the first protection layer 411 needs to be large enough tocover at least the element layer 420, the first protection layer 411 mayhave a larger thickness than the third protection layer 413.

As described in Structural Example 4, another protection layer may beprovided between the third protection layer 413 and the fourthprotection layer 414 so that mechanical strength of the light-emittingdevice 450 is increased.

Structural Example 6 of Light-Emitting Device

Here, a fifth protection layer that exhibits photocatalytic activity maybe provided on the outermost surface of the second protection layer 412or the outermost surface of the fourth protection layer 414. Inparticular, the fifth protection layer is preferably provided on thelight extraction side.

A light-emitting device 460 illustrated in FIG. 9 differs from thelight-emitting device 400 illustrated in FIG. 7 in that a fifthprotection layer 415 is provided in contact with the surface of thesecond protection layer 412, which does not face the element layer 420.

The fifth protection layer 415 includes a material that exhibitsphotocatalytic activity. Examples of the material are metal oxides suchas titanium oxide and zinc oxide.

The fifth protection layer 415 can be formed by a sputtering method, asol-gel method, a spin coating method, a dip method, a printing method,or the like.

An organic substance attached to a surface of the fifth protection layer415 can be decomposed by strong oxidation-reduction reaction of thefifth protection layer 415; thus, contamination of the surface of thelight-emitting device 460, which is caused by sebum or the like, can besuppressed. Furthermore, oil contamination is prevented from remainingby a super hydrophilic effect of the fifth protection layer 415; forexample, contamination of the surface of the light-emitting device 460can be easily removed only by exposing the surface to running water. Asa result, the number of times mechanical pressure is applied to thelight-emitting device 460, for example, by wiping off an organicsubstance attached or fixed on the surface of the light-emitting device460, can be reduced. Thus, a defect such as a crack formed on thesurface of the light-emitting device 460 or damage to an element forforming the element layer 420, such as the light-emitting element 410,can be suppressed.

<Formation Method of Element Layer>

A method for forming an element layer including a light-emitting elementover an insulating surface having flexibility will be described below.Note that description of the method for manufacturing the light-emittingdevice in any of the other embodiments can also be referred to for.

An element layer includes at least a light-emitting element and mayinclude elements other than the light-emitting element, such as a wiringelectrically connected to the light-emitting element and a transistorused for a circuit for controlling light emission of the light-emittingelement.

Here, a support provided with an insulating surface over which anelement layer is formed is called base material. For example, inStructural Example 4 and Structural Example 6, the substrate 401corresponds to the base material. In Structural Example 5, the stack ofthe fourth protection layer 414, the third protection layer 413, and theinsulating layer 409 corresponds to the base material.

As a method for forming an element layer over a base material providedwith an insulating surface having flexibility, there are a method inwhich an element layer is formed directly over a base material, and amethod in which an element layer is formed over a supporting basematerial that has a different stiffness from a base material, and thenthe element layer is separated from the supporting base material andtransferred to the base material.

In the case where a material of the base material can withstand heatingtemperature in the process for forming the element layer, it ispreferable that the element layer be formed directly over the basematerial, in which case a manufacturing process can be simplified. Atthis time, the element layer is preferably formed in a state where thebase material is fixed to the supporting base material, in which casetransfer of the element layer in a device and between devices can beeasy.

In the case of employing the method in which the element layer is formedover the supporting base material and then transferred to the basematerial, first, a separation layer (which corresponds to the separationlayer 503 in Embodiment 2, for example) and an insulating layer (whichcorresponds to the protection layer in Embodiment 2, for example) arestacked over a supporting base material (which corresponds to theformation substrate 501 in Embodiment 2, for example), and then theelement layer is formed over the insulating layer. Then, the elementlayer is separated from the supporting base material and thentransferred to the base material. At this time, a material is selectedso that separation at an interface between the supporting base materialand the separation layer, at an interface between the separation layerand the insulating layer, or in the separation layer occurs.

For example, it is preferable that a stacked layer of a layer includinga high-melting-point metal material, such as tungsten, and a layerincluding an oxide of the metal material be used as the separationlayer, and a stacked layer of a plurality of layers, such as a siliconnitride layer and a silicon oxynitride layer be used as the insulatinglayer over the separation layer. The use of the high-melting-point metalmaterial is preferable because the degree of freedom of the process forforming the element layer can be increased.

The separation may be performed by application of mechanical power,etching of the separation layer, by dripping of a liquid into part ofthe separation interface to penetrate the entire separation interface,or the like. Alternatively, separation may be performed by heating theseparation interface by utilizing a difference in coefficient of thermalexpansion.

Note that the above methods can be used in the case where a color filteror the like is formed over the base material having flexibility asdescribed in the following embodiment.

In the case of bonding a base material provided with an element layerand a base material provided with a color filter, the bonding ispreferably performed with the use of a curable material as the firstprotection layer 411. Alternatively, the bonding may be performedoutside a region where the first protection layer 411 is formed, withthe use of a curable material.

Alternatively, the following may be performed: a supporting basematerial provided with an element layer and a supporting base materialprovided with a color filter are bonded in advance, and the elementlayer and the color filter are separated from the respective supportingbase materials. By the bonding is performed in a state where the elementlayer and the color filter are formed over the respective supportingbase materials, alignment accuracy at the bonding can be improved.

<Materials and Formation Methods>

A material and a formation method that can be used for each componentdescribed above will be described below. Note that detailed explanationon the structure described in Embodiment 1 is not made. The materialsfor the components of the light-emitting device in any of the otherembodiments can be applied to the components of the light-emittingdevice of this embodiment.

[Substrate]

An organic resin, a glass material that is thin enough to haveflexibility, or the like can be used for the substrate 401. In the caseof a light-emitting device with a bottom emission structure or a dualemission structure, a material having a light-transmitting property withrespect to light emitted from the EL layer 405 is used for the substrate401.

Here, particularly in the case where a light-emitting device with a topemission structure, a conductive substrate including a metal or an alloymaterial is preferably used for the substrate 401, in which casedissipation of heat generated from a light-emitting element to be formedlater can be increased.

It is preferable to use a substrate subjected to insulation treatment insuch a manner that a surface of the conductive substrate is oxidized oran insulating film is formed on the surface. For example, an insulatingfilm may be formed over the surface of the conductive substrate by anelectrodeposition method, a coating method such as a spin coating methodor a dip method, a printing method such as a screen printing method, ora deposition method such as an evaporation method or a sputteringmethod. Alternatively, the surface of the substrate 401 may be oxidizedby a variety of methods, such as a method of exposing to or heating inan oxygen atmosphere or an anodic oxidation method.

In the case where the surface of the substrate 401 has projections, aplanarization layer may be provided in order to form a flat insulatingsurface which covers the projections. An insulating material, such as anorganic material or an inorganic material, can be used for theplanarization layer. The planarization layer can be formed by adeposition method such as a sputtering method, a coating method such asa spin-coating method or a dip method, a discharging method such as anink-jet method or a dispensing method, a printing method such as ascreen printing method, or the like.

[Light-Emitting Element]

The structure of the organic EL element 107 described in Embodiment 1can be applied to the structure of the light-emitting element 410.

[First Protection Layer and Third Protection Layer]

For the first protection layer and the third protection layer, a curablematerial, such as a photocurable organic resin, a curable resin of atwo-component-mixture type, or a thermosetting organic resin, a gel, orthe like can be used. For example, an epoxy resin, an acrylic resin, asilicone resin, a phenol resin, polyimide, polyvinyl chloride (PVC),polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA) can be used. Amaterial with low moisture permeability, such as an epoxy resin, isparticularly preferred.

Furthermore, a drying agent may be included in the first protectionlayer 411 and the third protection layer 413. As the drying agent, forexample, a substance which adsorbs moisture by chemical adsorption, suchas an oxide of an alkaline earth metal (e.g., calcium oxide or bariumoxide), can be used. Alternatively, a substance that adsorbs moisture byphysical adsorption, such as zeolite or silica gel, may be used as thedrying agent. In the case where the drying agent is applied to alighting device, when a granular drying agent is employed, light emittedfrom the light-emitting element 410 is diffusely reflected by the dryingagent; thus, a highly reliable light-emitting device with improvedviewing angle dependence can be achieved.

[Second Protection Layer and Fourth Protection Layer]

For the second protection layer and the fourth protection layer, amaterial that can be used for the flexible substrate described inEmbodiment 1 can be used, for example.

Alternatively, a material obtained by stacking a plurality of layers canbe used for the second protection layer and the fourth protection layer.For example, a material obtained by stacking two or more kinds of layersformed of an organic resin, a material obtained by stacking a layerformed of an organic resin and a layer formed of an inorganic material,or a material obtained by stacking two or more kinds of layers formed ofan inorganic material is used. In the case of using a stacked layer of aplurality of layers, a material having a higher hardness than at leastthe first protection layer or the third protection layer is selected asa material for a layer positioned on the surface side which does notface the light-emitting element 410. With a layer formed of an inorganicmaterial, moisture and the like are prevented from entering the insideof the light-emitting device, leading to improvement in the reliabilityof the light-emitting device.

As the inorganic material, an oxide material, a nitride material, or anoxynitride material of a metal or a semiconductor, or the like can beused. For example, silicon oxide, silicon nitride, silicon oxynitride,aluminum oxide, aluminum nitride, or aluminum oxynitride may be used.

For example, in the case where a layer formed of an organic resin and alayer formed of an inorganic material are stacked, the layer formed ofan inorganic material can be formed over or under the layer formed of anorganic resin by a sputtering method, a CVD method, a coating method, orthe like.

Alternatively, glass that is thin enough to have flexibility may be usedfor the the second protection layer and the fourth protection layer.Specifically, it is preferable to use a layer obtained by stacking anorganic resin layer, an adhesive layer, and a glass layer in this orderfrom the side close to the light-emitting element 410. The thickness ofthe glass layer is greater than or equal to 20 μm and less than or equalto 200 μm, preferably greater than or equal to 25 μm and less than orequal to 100 μm. With such a thickness, the glass layer can have both ahigh barrier property against water and oxygen and high flexibility. Thethickness of the organic resin layer is greater than or equal to 10 μmand less than or equal to 200 μm, preferably greater than or equal to 20μm and less than or equal to 50 μm. With such an organic resin layer incontact with the glass layer, occurrence of a break or a crack in theglass layer can be suppressed and mechanical strength can be improved.Such a composite material of a glass material and an organic resin isused for the second protection layer or the fourth protection layer,whereby a flexible light-emitting device having extremely highreliability can be achieved.

This embodiment can be implemented in appropriate combination with anyof the other embodiments described in this specification.

Embodiment 4

In this embodiment, a structural example of a display device will bedescribed as a specific example of the light-emitting device of oneembodiment of the present invention. Note that description of the sameportions as those in the above embodiments is omitted or simplified.

Structural Example 7 of Light-Emitting Device

In this structural example, a display device with a top emissionstructure will be described.

FIG. 10A is a schematic top view of a display device 270 described inthis structural example.

The display device 270 includes a display portion 271, a scan linedriver circuit 272, and a signal line driver circuit 273 over a topsurface of the third protection layer 413 over the fourth protectionlayer 414. The first protection layer 411 is provided to cover thedisplay portion 271 and the second protection layer 412 is provided overthe first protection layer 411. An external connection terminal 274 thatis electrically connected to the scan line driver circuit 272 and thesignal line driver circuit 273 is provided over the third protectionlayer 413. By an FPC 275 that is electrically connected to the externalconnection terminal 274, power supply potentials and signals, such asdriving signals, for driving the scan line driver circuit 272, thesignal line driver circuit 273, and the like can be externally input.

FIG. 10B is a schematic cross-sectional view along the line I-J and K-Lof FIG. 10A, which passes through regions including the externalconnection terminal 274, the scan line driver circuit 272, and thedisplay portion 271.

The insulating layer 409 is provided over the third protection layer413. The external connection terminal 274 and the element layer 420including the light-emitting element 410 and the scan line drivercircuit 272 (and the signal line driver circuit 273) are provided overthe insulating layer 409.

The external connection terminal 274 is formed using a conductive layerincluded in a transistor or a light-emitting element in the displaydevice 270. In this structural example, the external connection terminal274 includes a stack of a conductive layer used as gates of thetransistors and a conductive layer used as electrodes (source electrodesor drain electrodes). The external connection terminal 274 preferablyincludes a stack of a plurality of conductive layers in this mannerbecause the strength can be increased. A connector 276 is provided incontact with the external connection terminal 274. The FPC 275 iselectrically connected to the external connection terminal 274 throughthe connector 276. The connector 276 can be formed using a paste-form orsheet-form material that is obtained by mixing metal particles to athermosetting resin and exhibits anisotropic conductivity bythermocompression bonding. As the metal particles, particles in whichtwo or more kinds of metals are layered, for example, nickel particlescoated with gold are preferably used.

In FIG. 10B, the scan line driver circuit 272 includes an NMOS circuitin which transistors 211 and 212, which are n-channel transistors, areused in combination, as an example. The scan line driver circuit 272 isnot limited to an NMOS circuit and may have a variety of circuits suchas a CMOS circuit in which an n-channel transistor and a p-channeltransistor are used in combination or a PMOS circuit formed of p-channeltransistors. Note that the same applies to the signal line drivercircuit 273. Although a driver-integrated structure in which the scanline driver circuit 272 and the signal line driver circuit 273 areformed over an insulating surface provided with the display portion 271is described in this structural example, the scan line driver circuit272 or the signal line driver circuit 273, or both may be formed over asurface different from the insulating surface provided with the displayportion 271.

FIG. 10B illustrates a cross-sectional structure of one pixel as anexample of the display portion 271. The pixel includes a switchingtransistor 213, a current control transistor 214, and the firstelectrode 403 that is electrically connected to an electrode (a sourceelectrode or a drain electrode) of the current control transistor 214.An insulating layer 219 is provided to cover an end portion of the firstelectrode 403.

Note that there is no particular limitation on the structures of thetransistors included in the display portion 271, the scan line drivercircuit 272, and the signal line driver circuit 273.

The structure of the light-emitting element 410 in the above embodimentcan be applied to the structure of the light-emitting element 410 inthis embodiment. Since the display device 270 described in thisstructural example has a top emission structure, a light-transmittingmaterial is used for the second electrode 407, and a reflective materialis used for the first electrode 403.

Here, the insulating layer 409 can suppress dispersion of impuritiescontained in the third protection layer 413 and the fourth protectionlayer 414. It is preferable that an insulating layer 216 and aninsulating layer 218 each in contact with semiconductor layers of thetransistors suppress dispersion of an impurity into the semiconductorlayer. These insulating layers can be formed using, for example, oxideor nitride of a semiconductor such as silicon, or oxide or nitride ofmetal such as aluminum. Alternatively, a stack of such an inorganicinsulating material or a stack of such an inorganic insulating materialand an organic insulating material may be used. An insulating layer 217functions as a planarization film for reducing surface roughness causedby the transistors.

Here, a layer including the transistors and the light-emitting element410 corresponds to the element layer 420. In this structural example, astack of components from the top surface of the insulating layer 409 tothe second electrode 407 corresponds to the element layer 420.

An insulating layer 223 is provided on a surface of the secondprotection layer 412, which faces the light-emitting element 410. Acolor filter 221 is provided over the insulating layer 223 in theposition overlapping with the light-emitting element 410.

A material that does not easily permeate an impurity, such as water andoxygen, is preferably used for the insulating layer 223. A materialsimilar to that of the insulating layer 409 can be used for theinsulating layer 223.

A black matrix 222 is provided between adjacent color filters 221. Notethat the black matrix 222 may be provided in a region other than thedisplay portion 271; for example, the black matrix 222 may be providedin the scan line driver circuit 272.

An overcoat may be formed to cover the color filter 221 and the blackmatrix 222. The overcoat protects the color filter 221 and the blackmatrix 222 and suppresses the diffusion of impurities included in thecolor filter 221 and the black matrix 222.

The first protection layer 411 is provided over the element layer 420.The second protection layer 412 is provided over the first protectionlayer 411 with the insulating layer 223 provided therebetween. With sucha structure, damage to the element layer 420 by mechanical pressureexternally applied can be suppressed, whereby the display device 270with high reliability can be achieved.

The element layer 420 is provided over the third protection layer 413with the insulating layer 409 provided therebetween. The fourthprotection layer 414 is provided on the surface of the third protectionlayer 413, which does not face the element layer 420. Thus, themechanical strength on the rear surface of the display device 270 isincreased.

Accordingly, stress applied to the element layer 420 can be reduced andthe display device 270 itself can have high flexibility because thefirst protection layer 411 is formed to be sufficiently thin. With thesecond protection layer 412 which is provided on the outermost surfaceof the display device and has a high hardness at least at the surfacethat does not face the element layer 420, damage to elements for formingthe element layer 420, such as the light-emitting element 410, can besuppressed when the display device 270 is bent or mechanical pressure isexternally applied to the display device 270 even in the case where thefirst protection layer 411 is formed to be thin, whereby the displaydevice 270 can have high flexibility and high mechanical strength.

With the structure where the element layer 420 is sandwiched between astack of the first protection layer 411 and the second protection layer412 and a stack of the third protection layer 413 and the fourthprotection layer 414, damage to components of the element layer 420 dueto pressure from any plane of the display device 270 can be suppressed,whereby a highly reliable display device whose mechanical strength isimproved can be achieved.

Structural Example 8 of Light-Emitting Device

In this structural example, a display device with a bottom emissionstructure will be described. Note that description of the same portionsas those described in Structural Example 7 is omitted or simplified.

FIG. 11 is a schematic cross-sectional view of a display device 250described in this structural example.

The display device 250 differs from the display device 270 described inStructural Example 7 in that the position of the color filter 221 iscloser to the surface provided with the element layer 420 than thelight-emitting element 410.

In the light-emitting element 410, a light-transmitting material is usedfor the first electrode 403, and a reflective material is used for thesecond electrode 407. Accordingly, light from the EL layer 405 isemitted to the third protection layer 413 side.

Further, the color filter 221 is provided over the insulating layer 218covering transistors to overlap with the light-emitting element 410. Theinsulating layer 217 is provided to cover the color filter 221.

Here, for the insulating layer 217, the insulating layer 218, theinsulating layer 216, the insulating layer 409, the third protectionlayer 413, and the fourth protection layer 414, a material having alight-transmitting property with respect to light emitted from the ELlayer 405 is used.

The display device described in this embodiment has both highflexibility and high mechanical strength.

This embodiment can be implemented in appropriate combination with anyof the other embodiments described in this specification.

Embodiment 5

In this embodiment, as an example of the light-emitting device of oneembodiment of the present invention, a structural example of a lightingdevice to which the light-emitting element is applied will be describedwith reference to drawings. Note that description of the same portionsas those described in the above embodiments is omitted or simplified.

FIG. 12A is a schematic top view of a lighting device 290 described inthis embodiment. FIG. 12B is a schematic cross-sectional view takenalong a cutting plane line M-N in FIG. 12A. The lighting device 290 hasa top emission structure.

In the lighting device 290, the third protection layer 413 is formedover the fourth protection layer 414, and the light-emitting element 410is formed over the third protection layer 413 with the insulating layer409 provided therebetween. Furthermore, the first protection layer 411is provided to cover the light-emitting element 410, and the secondprotection layer 412 is provided over the first protection layer 411.Over the third protection layer 413, an extraction electrode 293electrically connected to the first electrode 403 of the light-emittingelement 410 and an extraction electrode 297 electrically connected tothe second electrode 407 of the light-emitting element 410 are providedin a region overlapping with neither the first protection layer 411 northe second protection layer 412.

Here, a structure including the light-emitting element 410 correspondsto the element layer 420.

For the first protection layer 411 and the second protection layer 412,a material having a light-transmitting property with respect to lightemitted from the EL layer 405 is used.

FIG. 12B illustrates an example in which the extraction electrode 293and the extraction electrode 297 are formed on the same plane surfaceand formed from the same layer as the first electrode 403. Here, part ofthe first electrode 403 forms the extraction electrode 293.

The second electrode 407 is formed to across an insulating layer 299that covers a step of the first electrode 403 and a step of theextraction electrode 297 to be in contact with the extraction electrode297, whereby the second electrode 407 is electrically connected to theextraction electrode 297.

Note that the extraction electrodes 293 and 297 may be formed with aconductive film different from the conductive film of the firstelectrode 403 in different steps. For example, a conductive film havinga larger thickness than the first electrode 403 may be placed below thelight-emitting element 410 (on the third protection layer 413 side) withan insulating layer provided therebetween. At this time, the conductivefilm preferably contains copper because the conductivity can beincreased. Alternatively, the first electrode 403 may be provided on andin contact with a top surface of the extraction electrode 293 withoutproviding the insulating layer.

The insulating layer 299 is provided to cover an end portion of thefirst electrode 403 to avoid short circuit between the second electrode407 and the first electrode 403. Moreover, an upper end portion or alower end portion of the insulating layer 299 preferably has a curvedsurface with a radius of curvature of 0.2 μm to 3 μm in order to beadequately covered with the second electrode 407 which is formed overthe insulating layer 299. The insulating layer 299 can be formed usingan organic compound, such as a negative photosensitive resin or apositive photosensitive resin, or an inorganic compound, such as siliconoxide, or silicon oxynitride.

As illustrated in FIGS. 12A and 12B, lens-shaped projections arepreferably formed on the surface of the second protection layer 412,which does not face the light-emitting element 410. The projections areformed for the purpose of suppressing the occurance of total reflectionfrom the light-emitting element 410 at an interface between the secondprotection layer 412 and the outside (the air), which results inreduction in light extraction efficiency. For the second protectionlayer 412, a lens array, a microlens array, a diffusion sheet, adiffusion film, or the like can be used. In particular, with the use ofa microlens array, the light extraction efficiency can be efficientlyimproved and the viewing angle dependence can also be improved; thus, alighting device with uniform light emission luminance can be achieved.

As a method for forming the projections on the surface of the secondprotection layer 412, a photolithography method, a nanoimprintingmethod, a sandblasting method, or the like can be used as appropriate.

Here, the refractive index of the second protection layer 412 ispreferably higher than or equal to the refractive index of the firstprotection layer 411. In other words, the reflective index is preferablyset such that a film positioned further from the light-emitting element410 has a higher refractive index. With such a structure, the occurrenceof total reflection at each interface of layers can be suppressed; thus,light emitted from the light-emitting element 410 can be almostthoroughly extracted.

Accordingly, stress applied to the element layer 420 can be reduced andthe lighting device 290 itself can have high flexibility because thefirst protection layer 411 is formed to be sufficiently thin. With thesecond protection layer 412 which is provided on the outermost surfaceof the lighting device and has a high hardness at least at the surfacethat does not face the element layer 420, damage to elements for formingthe element layer 420, such as the light-emitting element 410, can besuppressed when the lighting device 290 is bent or mechanical pressureis externally applied to the lighting device 290 even in the case wherethe first protection layer 411 is formed to be thin, whereby thelighting device 290 can have high flexibility and high mechanicalstrength.

With the structure where the element layer 420 is sandwiched between astack of the first protection layer 411 and the second protection layer412 and a stack of the third protection layer 413 and the fourthprotection layer 414, damage to components of the element layer 420 dueto pressure from any plane of the lighting device 290 can be suppressed,whereby a highly reliable lighting device whose mechanical strength isimproved can be achieved.

This embodiment can be implemented in appropriate combination with anyof the other embodiments described in this specification.

Embodiment 6

In this embodiment, a light-emitting device of one embodiment of thepresent invention will be described with reference to FIG. 13 and FIGS.14A and 14B.

Structural Example 9 of Light-Emitting Device

FIG. 13 is a schematic view of a light-emitting device of one embodimentof the present invention. The light-emitting device illustrated in FIG.13 includes a first substrate 901, a second substrate 902 which facesthe first substrate 901, and a light-emitting element 630 between thefirst substrate 901 and the second substrate 902. An adhesive layer 670is provided between the first substrate 901 and the second substrate902. A terminal portion 4601 includes two or more terminal electrodesover the third substrate 903. Part of the third substrate 903 issandwiched between the first substrate 901 and the second substrate 902.

The coefficient of thermal expansion of the third substrate 903 is assmall as 10 ppm/K. Thus, the third substrate 903 is less likely to bedeformed by heat applied to an anisotropic conductive film when an FPCis pressure-bonded to terminal electrodes 657 a and 657 b. This canprevent a crack in a resin and a wiring which are in contact with thethird substrate 903. Consequently, a light-emitting device with fewdisplay defects can be obtained.

In FIG. 13, the third substrate 903 does not overlap with thelight-emitting element 630. This means that a substrate that does nottransmit visible light can be used as the third substrate 903 and therange of material choices is expanded.

A difference in coefficient of thermal expansion between the firstsubstrate 901 and the second substrate 902 is within 10% of each of thecoefficient of thermal expansions of the substrates; thus, thesubstrates are not warped in one direction even in the case where aterminal electrode is heated.

Examples of materials that can be used for the light-emitting device ofone embodiment of the present invention will be described. Note that thematerials described in the above embodiments are not described here indetail. The materials for the components of the light-emitting device inany of the above embodiments can be applied to the components of thelight-emitting device of this embodiment.

[First Substrate, Second Substrate, and Third Substrate]

For the first substrate 901, the second substrate 902, and the thirdsubstrate 903, materials similar to the materials for the flexiblesubstrate described in Embodiment 1, the substrate, the secondprotection layer, and the fourth protection layer described inEmbodiment 3 can be used. The second substrate 902 may be a glasssubstrate, a quartz substrate, a sapphire substrate, a ceramicsubstrate, a metal substrate, or the like.

In particular, substrates whose coefficient of thermal expansion is 10ppm/K or lower are used as the first substrate 901 and the thirdsubstrate 903. The first substrate 901 and the third substrate 903 areless likely to be deformed by being heated. Thus, even when the firstsubstrate 901 and the third substrate 903 are heated, a crack is lesslikely to be generated in the first substrate 901 and the thirdsubstrate 903 themselves or the resin and the wiring which are incontact with the substrates.

[Terminal Electrode]

The terminal electrodes 657 a and 657 b can be formed with anelectrically conductive substance, such as a metal or a semiconductor.

[Adhesive Layer]

The adhesive layer 670 is in contact with the second electrode 622. Thesecond separation layer 601 b and the first substrate 901 are fixed bythe adhesive layer 670. The adhesive layer 670 can be formed with amaterial similar to that of the adhesive layer 109 described inEmbodiment 1. An adhesive with which the second substrate 902 and thelayer 505 can be chemically or physically separated when necessary, suchas an adhesive that is soluble in water or a solvent or an adhesivewhich is capable of being plasticized upon irradiation of UV light, isused for the adhesive layer 670.

[Organic EL Element]

The structure of the organic EL element 107 described in Embodiment 1can be applied to the organic EL element of this embodiment.

Structural Example 10 of Light-Emitting Device

FIG. 14A is a top view of a light-emitting device of one embodiment ofthe present invention. FIG. 14B is a cross-sectional view taken along acutting plane line P-Q in FIG. 14A.

A light-emitting device illustrated in FIG. 14B includes the terminalportion 4601, a pixel portion 4602, and a signal line circuit portion4603. The terminal portion 4601 includes a terminal electrode 657 overthe third substrate 903. In the pixel portion 4602, the light-emittingelement 630 and the transistor 650 for controlling the operation of thelight-emitting element are sandwiched between the first substrate 901and the second substrate 902. In the signal line circuit portion 4603,the transistor 652 is sandwiched between the first substrate 901 and thesecond substrate 902.

The transistor 650 which controls driving of a light-emitting elementand the transistor 652 which selects an intended pixel are formed ineach pixel.

The transistor 650 includes a gate electrode 606 formed over aninsulating layer 603, a gate insulating layer 608 formed over the gateelectrode 606, a semiconductor layer 610 formed over the gate insulatinglayer 608, and a source electrode 612 a and a drain electrode 612 bformed over the semiconductor layer 610. The transistor 650 is coveredwith a first insulating layer 614 and a second insulating layer 616. Thefirst electrode 618 is formed over the second insulating layer 616, alayer 620 containing an organic compound (EL layer) is formed over thefirst electrode layer 618, and the second electrode 622 is formed overthe layer 620 containing an organic compound.

The transistor 651 for controlling the operation of the light-emittingelement has a structure similar to that of the transistor 650. Note thatthe size (e.g., channel length and channel width) of each transistor andthe connection and the like of the transistors can be adjusted asappropriate.

The light-emitting element 630 is separated by the partition walls 624,whereby pixels are formed.

The partition wall 624 is provided so as to prevent disconnection of thelight-emitting element 630 due to steps of the opening and the likeprovided in the first electrode 618, the first insulating layer 614, andthe second insulating layer 616. Thus, it is preferable that thepartition wall 624 have a forward tapered shape so that a film formedover the partition wall 624 is prevented from being disconnected. In aforward tapered shape, a layer gradually increases in thickness and isin contact with a layer serving as a base.

The pixel portion 4602 and the signal line circuit portion 4603 includethe first substrate 901, a first organic layer 700 a, a first bufferlayer 604, and the insulating layer 603. The pixel portion 4602 and thesignal line circuit portion 4603 may further include a first separationlayer 601 a.

The second substrate 902 is provided with a second organic layer 700 b,a second buffer layer 662, and a passivation layer 663. The secondsubstrate 902 is further provided with a color filter 666 and alight-blocking film 664 which are in contact with the passivation layer663, and an overcoat 668 which covers the color filter 666 and thelight-blocking film 664. Note that the second separation layer 601 b maybe provided over the second substrate 902 as long as the secondseparation layer 601 b does not block light emitted from thelight-emitting element 630.

The coefficient of thermal expansion of the third substrate 903, whichoverlaps with the terminal portion 4601 of the light-emitting device, isas small as 10 ppm/K. Thus, the third substrate 903 is less likely to bedeformed by heat applied to an anisotropic conductive film when an FPCis pressure-bonded to the terminal electrode 657. This can prevent acrack in a resin and a wiring which are in contact with the thirdsubstrate 903. Consequently, a light-emitting device with few displaydefects can be obtained.

In the structure of FIG. 14B, there is no need to provide the thirdsubstrate 903 in the pixel portion 4602. This means that a substratethat does not transmit visible light can be used as the third substrate903.

A difference in coefficient of thermal expansion between the firstsubstrate 901 and the second substrate 902 is within 10% of each of thecoefficient of thermal expansions of the substrates; thus, thesubstrates are not warped in one direction even in the case where theterminal electrode 657 is heated.

A material similar to that for the separation layer described inEmbodiment 2 can be used for the first separation layer 601 a and thesecond separation layer 601 b. A material similar to that for theadhesive layer described in Embodiment 2 can be used for each of thefirst organic layer 700 a and the second organic layer 700 b. A materialsimilar to that for the protection layer described in Embodiment 2 canbe used for each of the first buffer layer 604 and the second bufferlayer 662.

In addition, materials similar to the base film 121, the gate electrode131, the gate insulating film 133, the semiconductor layer 135, theconductive layer 137 a, and the conductive layer 137 b which aredescribed in Embodiment 1 can be used for the insulating layer 603, thegate electrode 606, the gate insulating layer 608, the semiconductorlayer 610, the source electrode 612 a, and the drain electrode 612 b,respectively.

The first insulating layer 614 is formed over the semiconductor layer610, the source electrode 612 a, and the drain electrode 612 b. Amaterial similar to that of the insulating film 141 described inEmbodiment 1 can be used for the first insulating layer 614.

The second insulating layer 616 is formed over the first insulatinglayer 614. A material similar to that of the planarization layer 105described in Embodiment 1 can be used for the second insulating layer616.

The partition wall 624 is provided to avoid electrical short circuitbetween adjacent first electrodes 618. In the case where a metal mask isused for forming the layer 620 containing an organic compound, thepartition wall 624 also has a function of dividing the layer 620containing an organic compound between the light-emitting elements. Amaterial similar to that of the partition wall 161 described inEmbodiment 1 can be used for the partition wall 624.

Materials similar to those of the base film 185, the black matrix 183,the color filters 157 and 181, the overcoat which are described inEmbodiment 2 can be used for the passivation layer 663, thelight-blocking film 664, the color filter 666, and the overcoat 668.

<Manufacturing Method 3 of Light-Emitting Device>

A method for manufacturing the light-emitting device illustrated inFIGS. 14A and 14B will be described with reference to FIGS. 16A to 16D.Note that the same portions as those described in Embodiment 2 are notdescribed here in detail.

First, the first separation layer 601 a is formed over the formationsubstrate 501, and the layer 505 is formed over the first separationlayer 601 a (FIG. 16A).

There is no particular limitation on a layer formed as the layer 505.Here, a protection layer, the terminal electrode 657, a transistor, thelight-emitting element 630, and the like are formed as the layer 505.

Next, the layer 505 and the second substrate 902 are bonded with theadhesive layer 670, and the layer 505 is separated from the formationsubstrate 501 along the first separation layer 601 a. Accordingly, thelayer 505 is placed on the second substrate 902 side (FIG. 16B). Notethat necessary components, such as a color filter, are provided inadvance over the second substrate 902.

Next, with the use of the first organic layer 700 a, the third substrate903 is bonded to the first separation layer 601 a exposed by theseparation of the layer 505 from the formation substrate 501 or thelayer 505 in which the protection layer is exposed by the separation ofthe layer 505 from the formation substrate 501 such that the thirdsubstrate 903 overlaps with the terminal electrode 657 (FIG. 16C).

Then, the first substrate 901 is attached to the first organic layer 700a not to overlap with the terminal electrode 657 (FIG. 16D).

Through the above steps, the layer 505 provided with the transistor andthe light-emitting element 630 can be formed over the first substrate901.

Finally, an FPC 4605 is attached to each electrode of the terminalelectrode 657 with the use of an anisotropic conductive member. An ICchip or the like may be mounted if necessary.

In the above manner, the light-emitting device illustrated in FIGS. 14Aand 14B can be manufactured.

Structural Example 11 of Light-Emitting Device

In the light-emitting device of one embodiment of the present invention,the structure of the terminal portion 4601 which differs from that inStructural Example 10 will be described with reference to FIG. 15.

Note that the same reference numerals are used for the same parts asthose of the light-emitting device described in Embodiment 1, anddescription of the parts with the same reference numerals will beomitted here.

In a light-emitting device illustrated in FIG. 15, the terminal portion4601 is provided between the first substrate 901 and the second organiclayer 700 b.

The terminal electrode 657 overlaps with the first substrate 901 havinga small coefficient of thermal expansion. Thus, even when the terminalelectrode 657 is heated, the first substrate 901 is less likely to bedeformed by heat, so that a crack is less likely to be generated in theresin and the wiring which are in contact with the first substrate 901.

With the above structure, since the first substrate 901 is less likelyto be deformed by heat even when the terminal electrode 657 is heated, acrack is less likely to be generated in the first substrate 901 itselfor the resin and the wiring which are in contact with the substrate.Consequently, a light-emitting device without display defects can beobtained.

This embodiment can be freely combined with other embodiments.

Embodiment 7

In this embodiment, an example of a light-emitting element which can beapplied to one embodiment of the present invention will be describedwith reference to FIGS. 17A to 17F.

Each of the light-emitting elements shown in this embodiment includes apair of electrodes (a first electrode and a second electrode) and an ELlayer(s) provided between the pair of electrodes. One of the electrodesserves as an anode and the other serves as a cathode. The EL layer hasat least a light-emitting layer.

A light-emitting element illustrated in FIG. 17A includes an EL layer203 between a first electrode 201 and a second electrode 205. In thisembodiment, the first electrode 201 serves as the anode, and the secondelectrode 205 serves as the cathode.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 201 and the secondelectrode 205, holes are injected to the EL layer 203 from the firstelectrode 201 side and electrons are injected to the EL layer 203 fromthe second electrode 205 side. The injected electrons and holes arerecombined in the EL layer 203 and a light-emitting material containedin the EL layer 203 emits light.

The EL layer 203 includes at least a light-emitting layer. In additionto the light-emitting layer, the EL layer 203 may further include one ormore layers containing any of a material with a high hole-injectionproperty, a material with a high hole-transport property, ahole-blocking material, a material with a high electron-transportproperty, a material with a high electron-injection property, a bipolarproperty (a material with a high electron- and hole-transport property),and the like.

Either a low molecular compound or a high molecular compound can be usedfor the EL layer 203, and an inorganic compound may also be used.

A specific example of a structure of the EL layer 203 is illustrated inFIG. 17B. In the EL layer 203 illustrated in FIG. 17B, a hole-injectionlayer 301, a hole-transport layer 302, a light-emitting layer 303, anelectron-transport layer 304, and an electron-injection layer 305 arestacked in this order from the first electrode 201 side.

A light-emitting element illustrated in FIG. 17C includes the EL layer203 between the first electrode 201 and the second electrode 205, andfurther includes an intermediate layer 207 between the EL layer 203 andthe second electrode 205.

A specific example of a structure of the intermediate layer 207 isillustrated in FIG. 17D. The intermediate layer 207 includes at least acharge-generation region 308. In addition to the charge-generationregion 308, the intermediate layer 207 may further include anelectron-relay layer 307 and an electron-injection buffer layer 306.

When a voltage higher than the threshold voltage of the light-emittingelement is applied between the first electrode 201 and the secondelectrode 205, holes and electrons are generated in thecharge-generation region 308, and the holes move into the secondelectrode 205 and the electrons move into the electron-relay layer 307.The electron-relay layer 307 has a high electron-transport property andimmediately transfers the electrons generated in the charge-generationregion 308 to the electron-injection buffer layer 306. Theelectron-injection buffer layer 306 reduces a barrier to electroninjection into the EL layer 203, so that the efficiency of the electroninjection into the EL layer 203 is increased. Thus, the electronsgenerated in the charge-generation region 308 are injected into the LUMOlevel of the EL layer 203 through the electron-relay layer 307 and theelectron-injection buffer layer 306.

In addition, the electron-relay layer 307 can prevent reaction at theinterface between a material contained in the charge-generation region308 and a material contained in the electron-injection buffer layer 306.Thus, it is possible to prevent interaction such as damaging thefunctions of the charge-generation region 308 and the electron-injectionbuffer layer 306.

As illustrated in light-emitting elements in FIGS. 17E and 17F, aplurality of EL layers may be stacked between the first electrode 201and the second electrode 205. In this case, the intermediate layer 207is preferably provided between the stacked EL layers. For example, thelight-emitting element illustrated in FIG. 17E includes the intermediatelayer 207 between a first EL layer 203 a and a second EL layer 203 b.The light-emitting element illustrated in FIG. 17F includes n EL layers(n is a natural number of 2 or more), and the intermediate layers 207between the EL layers.

The following will show behaviors of electrons and holes in theintermediate layer 207 between the EL layer 203(m) and the EL layer203(m+1). When a voltage higher than the threshold voltage of thelight-emitting element is applied between the first electrode 201 andthe second electrode 205, holes and electrons are generated in theintermediate layer 207, and the holes move into the EL layer 203(m+1)provided on the second electrode 205 side and the electrons move intothe EL layer 203(m) provided on the first electrode 201 side. The holesinjected into the EL layer 203(m+1) are recombined with the electronsinjected from the second electrode 205 side, so that a light-emittingmaterial contained in the EL layer 203(m+1) emits light. Further, theelectrons injected into the EL layer 203(m) are recombined with theholes injected from the first electrode 201 side, so that alight-emitting material contained in the EL layer 203(m) emits light.Thus, the holes and electrons generated in the intermediate layer 207cause light emission in the respective EL layers.

Note that the EL layers can be provided in contact with each other aslong as the same structure as the intermediate layer is formedtherebetween. For example, when the charge-generation region is formedover one surface of an EL layer, another EL layer can be provided incontact with the surface.

Further, by forming EL layers to emit light of different colors fromeach other, a light-emitting element as a whole can provide lightemission of a desired color. For example, by forming a light-emittingelement having two EL layers such that the emission color of the firstEL layer and the emission color of the second EL layer are complementarycolors, the light-emitting element can provide white light emission as awhole. Note that the word “complementary” means color relationship inwhich an achromatic color is obtained when colors are mixed. That is,white light emission can be obtained by mixture of light from materialswhose emission colors are complementary colors. This can be applied to alight-emitting element having three or more EL layers.

FIGS. 17A to 17F can be used in an appropriate combination. For example,the intermediate layer 207 can be provided between the second electrode205 and the EL layer 203(n) in FIG. 17F.

Examples of materials which can be used for each layer will be describedbelow. Note that each layer is not limited to a single layer, but may bea stack of two or more layers.

<Anode>

The electrode serving as the anode (the first electrode 201) can beformed using one or more kinds of conductive metals, alloys, conductivecompounds, and the like. In particular, it is preferable to use amaterial with a high work function (4.0 eV or more). Examples includeITO, indium tin oxide containing silicon or silicon oxide, indium zincoxide, indium oxide containing tungsten oxide and zinc oxide, graphene,gold, platinum, nickel, tungsten, chromium, molybdenum, iron, cobalt,copper, palladium, and a nitride of a metal material (e.g., titaniumnitride).

When the anode is in contact with the charge-generation region, any of avariety of conductive materials can be used regardless of their workfunctions; for example, aluminum, silver, an alloy containing aluminum,or the like can be used.

<Cathode>

The electrode serving as the cathode (the second electrode 205) can beformed using one or more kinds of conductive metals, alloys, conductivecompounds, and the like. In particular, it is preferable to use amaterial with a low work function (3.8 eV or less). Examples includealuminum, silver, an element belonging to Group 1 or 2 of the periodictable (e.g., an alkali metal such as lithium or cesium, an alkalineearth metal such as calcium or strontium, or magnesium), an alloycontaining any of these elements (e.g., Mg—Ag or Al—Li), a rare earthmetal such as europium or ytterbium, and an alloy containing any ofthese rare earth metals.

Note that in the case where the cathode is in contact with thecharge-generation region, a variety of conductive materials can be usedregardless of its work function. For example, ITO, silicon, or indiumtin oxide containing silicon oxide can be used.

The light-emitting element may have a structure in which one of theanode and the cathode is formed using a conductive film that transmitsvisible light and the other is formed using a conductive film thatreflects visible light, or a structure in which both the anode and thecathode are formed using conductive films that transmit visible light.

The conductive film that transmits visible light can be formed using,for example, indium oxide, ITO, indium zinc oxide, zinc oxide, or zincoxide to which gallium is added. Alternatively, a film of a metalmaterial such as gold, silver, platinum, magnesium, nickel, tungsten,chromium, molybdenum, iron, cobalt, copper, palladium, or titanium; analloy containing any of these metal materials; or a nitride of any ofthese metal materials (e.g., titanium nitride) can be formed thin so asto have a light-transmitting property. Alternatively, a stack of any ofthe above materials can be used as the conductive layer. For example, astacked film of ITO and an alloy of silver and magnesium is preferablyused, in which case conductivity can be increased. Furtheralternatively, graphene or the like may be used.

For the conductive film that reflects visible light, for example, ametal material, such as aluminum, gold, platinum, silver, nickel,tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or analloy including any of these metal materials can be used. Lanthanum,neodymium, germanium, or the like may be added to the metal material orthe alloy. Furthermore, an alloy containing aluminum (an aluminum alloy)such as an alloy of aluminum and titanium, an alloy of aluminum andnickel, or an alloy of aluminum and neodymium; or an alloy containingsilver such as an alloy of silver and copper, an alloy of silver,copper, and palladium, or an alloy of silver and magnesium can be usedfor the conductive film. An alloy of silver and copper is preferablebecause of its high heat resistance. Moreover, a metal film or a metaloxide film is stacked on an aluminum alloy film, whereby oxidation ofthe aluminum alloy film can be suppressed. Examples of a material forthe metal film or the metal oxide film include titanium and titaniumoxide. Alternatively, the conductive film having a property of propertyof transmitting visible light and a film containing any of the abovemetal materials may be stacked. For example, a stacked film of silverand ITO or a stacked film of an alloy of silver and magnesium and ITOcan be used.

The electrodes may be formed separately by an evaporation method or asputtering method. Alternatively, a discharging method such as anink-jet method, a printing method such as a screen printing method, or aplating method may be used.

Note that in the case where a conductive oxide film is formed as theabove conductive film having a property of transmitting visible light bya sputtering method, the use of a deposition atmosphere containing argonand oxygen allows the light-transmitting property to be increased.

Furthermore, in the case where the conductive oxide film is formed overthe EL layer, it is preferable to stack a first conductive oxide filmformed under an atmosphere containing argon with a reduced oxygenconcentration and a second conductive oxide film formed under anatmosphere containing argon and oxygen because damage to the EL layercaused by the film formation can be reduced. Here, the purity of anargon gas used for formation of the first conductive oxide film ispreferably high, and for example, it is preferable to use the argon gaswhose dew point is lower than or equal to −70° C., more preferably lowerthan or equal to −100° C.

<Hole-Injection Layer 301>

The hole-injection layer 301 contains a material with a highhole-injection property.

Examples of the substance having a high hole-injection property includemetal oxides such as molybdenum oxide, vanadium oxide, ruthenium oxide,tungsten oxide, and manganese oxide; and phthalocyanine-based compoundssuch as phthalocyanine (abbreviation: H₂Pc) and copper(II)phthalocyanine (abbreviation: CuPc).

Other examples of the substance having a high hole-injection propertyinclude high molecular compounds such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(-vinyltriphenylamine) (abbreviation: PVTPA);and high molecular compounds to which acid such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonic acid) (PEDOT/PSS)is added.

The hole-injection layer 301 may serve as the charge-generation region.When the hole-injection layer 301 in contact with the anode serves asthe charge-generation region, a variety of conductive materials can beused for the anode regardless of their work functions. Materialscontained in the charge-generation region will be described later.

<Hole-Transport Layer 302>

The hole-transport layer 302 contains a material with a highhole-transport property.

The substance having a high hole-transport property is a substance whosehole-transport property is higher than its electron-transport property;particularly, it is preferable that the hole mobility of the substancehaving a high hole-transport property be greater than or equal to 10⁻⁶cm²/Vs. A variety of compounds can be used. For example, an aromaticamine compound such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD) or4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation: BPAFLP);a carbazole derivative such as 4,4′-di(N-carbazolyl)biphenyl(abbreviation: CBP), 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole(abbreviation: CzPA), or9-phenyl-3-[4-(10-phenyl-9-anthryl)phenyl]-9H-carbazole (abbreviation:PCzPA); aromatic hydrocarbon compound such as2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation: t-BuDNA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA), or9,10-diphenylanthracene (abbreviation: DPAnth); a high molecularcompound such as PVK or PVTPA.

<Light-Emitting Layer 303>

For the light-emitting layer 303, a fluorescent compound which exhibitsfluorescence or a phosphorescent compound which exhibits phosphorescencecan be used.

Examples of the fluorescent compound that can be used for thelight-emitting layer 303 includeN,N′-bis[4-(9H-carbazol-9-yl)phenyl]-N,N-diphenyl stilbene-4,4′-diamine(abbreviation: YGA2S), N-(9,10-diphenyl-2-anthryl)-N,9-diphenyl-9H-carbazol-3-amine (abbreviation: 2PCAPA), and rubrene.

Examples of the phosphorescent compound that can be used for thelight-emitting layer 303 include organometallic complexes such asbis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic), tris(2-phenylpyridinato-N,C^(2′))iridium(III)(abbreviation: Ir(ppy)₃), and(acetylacetonato)bis(3,5-dimethyl-2-phenylpyrazinato)iridium(III)(abbreviation: Ir(mppr-Me)₂(acac)).

The light-emitting layer 303 may have a structure in which any of theabove-described light-emitting organic compounds (a light-emittingsubstance or a guest material) is dispersed in another substance (a hostmaterial). As the host material, a variety of kinds of materials can beused, and it is preferable to use a substance which has a lowestunoccupied molecular orbital level (LUMO level) higher than that of theguest material and has a highest occupied molecular orbital level (HOMOlevel) lower than that of the guest material.

With the structure in which the guest material is dispersed in the hostmaterial, crystallization of the light-emitting layer 303 can besuppressed. Furthermore, concentration quenching due to highconcentration of the guest material can be suppressed.

As the host material, the above-described substance having a highhole-transport property (e.g., an aromatic amine compound or a carbazolederivative) or a later-described substance having a highelectron-transport property (e.g., a metal complex having a quinolineskeleton or a benzoquinoline skeleton or a metal complex having anoxazole-based or thiazole-based ligand) can be used. As the hostmaterial, specifically, a metal complex such astris(8-quinolinolato)aluminum(III) (abbreviation: Alq) orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq); a heterocyclic compound such as3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen), orbathocuproine (abbreviation: BCP); a condensed aromatic compound such asCzPA, DNA, t-BuDNA, or DPAnth; or an aromatic amine compound such as NPBcan be used.

Alternatively, as the host material, plural kinds of materials can beused. For example, in order to suppress crystallization, a substancesuch as rubrene that suppresses crystallization, may be further added.In addition, NPB, Alq, or the like may be further added in order totransfer energy to the guest material more efficiently.

Further, when a plurality of light-emitting layers are provided andemission colors of the layers are made different, light emission of adesired color can be obtained from the light-emitting element as awhole. For example, the emission colors of first and secondlight-emitting layers are complementary in a light-emitting elementhaving the two light-emitting layers, so that the light-emitting elementcan be made to emit white light as a whole. Further, the same applies toa light-emitting element having three or more light-emitting layers.

<Electron-Transport Layer 304>

The electron-transport layer 304 contains a material with a highelectron-transport property.

The material with a high electron-transport property is preferably anorganic compound having a property of transporting more electrons thanholes, and is especially preferably a material with an electron mobilityof 10⁻⁶ cm²/V·s or more.

As the substance having a high electron-transport property, for example,a metal complex having a quinoline skeleton or a benzoquinolineskeleton, such as Alq or Balq, can be used. Alternatively, a metalcomplex having an oxazole-based ligand or a thiazole-based ligand, suchas bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂) orthe like can be used. Alternatively, TAZ, BPhen, BCP, or the like can beused.

<Electron-Injection Layer 305>

The electron-injection layer 305 contains a material with a highelectron-injection property.

Examples of the substance having a high electron-injection propertyinclude alkali metals, alkaline earth metals, and compounds thereof,such as lithium, cesium, calcium, lithium fluoride, cesium fluoride,calcium fluoride, and lithium oxide. In addition, a rare earth metalcompound such as erbium fluoride can also be used. Furthermore, thesubstance for the electron-transport layer 304 described above can alsobe used.

<Charge-Generation Region>

The charge-generation region included in the hole-injection layer andthe charge-generation region 308 each contains a material with a highhole-transport property and an acceptor material (electron acceptor).Note that the acceptor material is preferably added so that the massratio of the acceptor material to the material with a highhole-transport property is 0.1:1 to 4.0:1.

The charge-generation region is not limited to a structure in which amaterial with a high hole-transport property and an acceptor materialare contained in the same film, and may have a structure in which alayer containing a material with a high hole-transport property and alayer containing an acceptor material are stacked. Note that in the caseof a stacked-layer structure in which the charge-generation region isprovided on the cathode side, the layer containing the material with ahigh hole-transport property is in contact with the cathode, and in thecase of a stacked-layer structure in which the charge-generation regionis provided on the anode side, the layer containing the acceptormaterial is in contact with the anode.

The material with a high hole-transport property is preferably anorganic compound having a property of transporting more holes thanelectrons, and is especially preferably an organic compound with a holemobility of 10⁻⁶ cm²/V·s or more.

Specifically, it is possible to use any of the materials with a highhole-transport property shown as materials that can be used for thehole-transport layer 302, such as aromatic amine compounds such as NPBand BPAFLP, carbazole derivatives such as CBP, CzPA, and PCzPA, aromatichydrocarbon compounds such as t-BuDNA, DNA, and DPAnth, and highmolecular compounds such as PVK and PVTPA.

Examples of the acceptor material include organic compounds, such as7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ) and chloranil, oxides of transition metals, and oxides ofmetals that belong to Groups 4 to 8 in the periodic table. Specifically,vanadium oxide, niobium oxide, tantalum oxide, chromium oxide,molybdenum oxide, tungsten oxide, manganese oxide, and rhenium oxide arepreferable since their electron-accepting property is high. Inparticular, use of molybdenum oxide is preferable because of itsstability in the atmosphere, a low hygroscopic property, and easilyhandling.

<Electron-Injection Buffer Layer 306>

The electron-injection buffer layer 306 contains a material with a highelectron-injection property. The electron-injection buffer layer 306facilitates electron injection from the charge-generation region 308into the EL layer 203. As the material having a high electron-injectionproperty, any of the above-described materials can be used.Alternatively, the electron-injection buffer layer 306 may contain anyof the above-described materials with a high electron-transport propertyand donor materials.

<Electron-Relay Layer 307>

The electron-relay layer 307 immediately accepts electrons drawn out ofthe acceptor material in the charge-generation region 308.

The electron-relay layer 307 contains a material with a highelectron-transport property. As the material with a highelectron-transport property, a phthalocyanine-based material or a metalcomplex having a metal-oxygen bond and an aromatic ligand is preferablyused.

As the phthalocyanine-based material, specifically, it is possible touse CuPc, vanadyl 2,9,16,23-tetraphenoxy-29H,31H-phthalocyanine(PhO-VOPc), or the like.

As the metal complex having a metal-oxygen bond and an aromatic ligand,a metal complex having a metal-oxygen double bond is preferably used. Ametal-oxygen double bond has an acceptor property; thus, electrons cantransfer (be donated and accepted) more easily.

As the metal complex having a metal-oxygen bond and an aromatic ligand,a phthalocyanine-based material is also preferably used.

Note that as the phthalocyanine-based materials described above, aphthalocyanine-based material having a phenoxy group is preferable.Specifically, a phthalocyanine derivative having a phenoxy group, suchas PhO-VOPc, is preferable. The phthalocyanine derivative having aphenoxy group is soluble in a solvent; thus, the phthalocyaninederivative has an advantage of being easily handled during formation ofa light-emitting element and an advantage of facilitating maintenance ofan apparatus used for film formation.

The electron-relay layer 307 may further contain any of theabove-described donor materials. When the donor material is contained inthe electron-relay layer 307, electrons can transfer easily and thelight-emitting element can be driven at a lower voltage.

The LUMO levels of the material with a high electron-transport propertyand the donor material are preferably −5.0 eV to −3.0 eV, i.e., betweenthe LUMO level of the acceptor material contained in thecharge-generation region 308 and the LUMO level of the material with ahigh electron-transport property contained in the electron-transportlayer 304 (or the LUMO level of the EL layer 203 in contact with theelectron-relay layer 307 or the electron-injection buffer layer 306).When a donor material is contained in the electron-relay layer 307, asthe material with a high electron-transport property, a material with aLUMO level higher than the acceptor level of the acceptor materialcontained in the charge-generation region 308 can be used.

The above-described layers included in the EL layer 203 and theintermediate layer 207 can be formed separately by any of the followingmethods: an evaporation method (including a vacuum evaporation method);a transfer method; a printing method; a discharging method such as anink-jet method or a dispensing method; a coating method such as aspin-coating method; and the like.

This embodiment can be freely combined with other embodiments.

Embodiment 8

In this embodiment, an oxide semiconductor which can be used for thesemiconductor layer of a transistor included in a light-emitting deviceof one embodiment of the present invention will be described.

At least indium (In) or zinc (Zn) is preferably contained as an oxidesemiconductor used for the semiconductor layer of the transistor. Inparticular, In and Zn are preferably contained. A stabilizer forstrongly bonding oxygen is preferably contained in addition to In andZn. As the stabilizer, at least one of gallium (Ga), tin (Sn), zirconium(Zr), hafnium (Hf), and aluminum (Al) may be contained.

As another stabilizer, one or plural kinds of lanthanoid such aslanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy),holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium(Lu) may be contained.

As the oxide semiconductor, the following can be used, for example: anIn—Sn—Ga—Zn-based oxide, an In—Ga—Zn-based oxide, an In—Sn—Zn-basedoxide, an In—Zr—Zn-based oxide, an In—Al—Zn-based oxide, aSn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, an In—Lu—Zn-based oxide, an In—Zn-based oxide, aSn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide, anIn-based oxide, a Sn-based oxide, or a Zn-based oxide.

Note that here, for example, an “In—Ga—Zn-based oxide” refers to anoxide mainly containing In, Ga, and Zn, and there is no limitation onthe ratio of In to Ga and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0) may beused as the oxide semiconductor. Note that M represents one or moremetal elements selected from Ga, Fe, Mn, or Co. Still alternatively, amaterial represented by In₂SnO₅(ZnO)_(n) (n>0 is satisfied, and n is aninteger) may be used as an oxide semiconductor.

For example, an In—Ga—Zn-based oxide with an atomic ratio ofIn:Ga:Zn=3:1:2, 1:1:1, or 2:2:1, or an oxide whose atomic ratio is inthe neighborhood of the above atomic ratios can be used. Alternatively,an In—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1,In:Sn:Zn=2:1:3, or In:Sn:Zn=2:1:5, or an oxide with an atomic ratioclose to the above atomic ratios may be used.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy a formula (a−A)²+(b−B)²+(c−C)²≤r². In the formula, r maybe, for example, 0.05. The same applies to other oxides.

However, the composition of the oxide semiconductor is not limited tothose described above, and an oxide semiconductor having an appropriatecomposition may be used depending on necessary semiconductorcharacteristics (e.g., field-effect mobility or threshold voltage). Inorder to obtain the required semiconductor characteristics, it ispreferable that the carrier concentration, the impurity concentration,the defect density, the atomic ratio between a metal element and oxygen,the interatomic distance, the density, and the like be set toappropriate values.

When an oxide semiconductor is highly purified, the off-state current ofa transistor using such an oxide semiconductor in a semiconductor layercan be sufficiently reduced (here, the off-state current means a draincurrent when a potential difference between a source and a gate is equalto or lower than the threshold voltage in the off state, for example). Ahighly purified oxide semiconductor can be obtained, for example, insuch a manner that a film is deposited while heating is performed so asto prevent hydrogen and a hydroxyl group from being contained in theoxide semiconductor, or heat treatment is performed after filmdeposition so as to remove hydrogen and a hydroxyl group from the film.When an In—Ga—Zn-based oxide which is used for a channel region of atransistor is highly purified, the off-state current per channel widthcan be approximately from 1×10⁻²⁴ A/μm (1 yA/μm) to 1×10⁻²² A/μm (100yA/μm).

An oxide semiconductor film may be in a non-single-crystal state, forexample. The non-single-crystal state is, for example, structured by atleast one of c-axis aligned crystal (CAAC), polycrystal, microcrystal,and an amorphous part. The density of defect states of an amorphous partis higher than those of microcrystal and CAAC. The density of defectstates of microcrystal is higher than that of CAAC. Note that an oxidesemiconductor including CAAC is referred to as a CAAC-OS (c-axis alignedcrystalline oxide semiconductor).

For example, the oxide semiconductor film may include a CAAC-OS. In theCAAC-OS, for example, c-axes are aligned, and a-axes and/or b-axes arenot macroscopically aligned.

For example, an oxide semiconductor film may include microcrystal. Notethat an oxide semiconductor including microcrystal is referred to as amicrocrystalline oxide semiconductor. A microcrystalline oxidesemiconductor film includes microcrystal (also referred to asnanocrystal) with a size greater than or equal to 1 nm and less than 10nm, for example.

For example, an oxide semiconductor film may include an amorphous part.Note that an oxide semiconductor including an amorphous part is referredto as an amorphous oxide semiconductor. An amorphous oxide semiconductorfilm, for example, has disordered atomic arrangement and no crystallinecomponent. Alternatively, an amorphous oxide semiconductor film is, forexample, absolutely amorphous and has no crystal part.

Note that an oxide semiconductor film may be a mixed film including anyof a CAAC-OS, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor. The mixed film, for example, includes a region ofan amorphous oxide semiconductor, a region of a microcrystalline oxidesemiconductor, and a region of a CAAC-OS. Further, the mixed film mayhave a stacked structure including a region of an amorphous oxidesemiconductor, a region of a microcrystalline oxide semiconductor, and aregion of a CAAC-OS, for example.

Note that an oxide semiconductor film may be in a single-crystal state,for example.

An oxide semiconductor film preferably includes a plurality of crystalparts. In each of the crystal parts, a c-axis is preferably aligned in adirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film. Note that, among crystal parts, the directionsof the a-axis and the b-axis of one crystal part may be different fromthose of another crystal part. An example of such an oxide semiconductorfilm is a CAAC-OS film.

Note that in most cases, a crystal part in the CAAC-OS film fits insidea cube whose one side is less than 100 nm. In an image obtained with atransmission electron microscope (TEM), a boundary between crystal partsin the CAAC-OS film is not clearly detected. Further, with the TEM, agrain boundary in the CAAC-OS film is not clearly found. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis isaligned in a direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film. Further, in each of the crystal parts, metal atoms arearranged in a triangular or hexagonal configuration when seen from thedirection perpendicular to the a-b plane, and metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms are arranged in alayered manner when seen from the direction perpendicular to the c-axis.Note that, among crystal parts, the directions of the a-axis and theb-axis of one crystal part may be different from those of anothercrystal part. In this specification, a term “perpendicular” includes arange from 80° to 100°, preferably from 85° to 95°. In addition, a term“parallel” includes a range from −10° to 10°, preferably from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm deposition is accompanied with the formation of the crystal partsor followed by the formation of the crystal parts throughcrystallization treatment such as heat treatment. Hence, the c-axes ofthe crystal parts are aligned in the direction parallel to a normalvector of the surface where the CAAC-OS film is formed or a normalvector of the surface of the CAAC-OS film.

With the use of the CAAC-OS film in a transistor, change in electriccharacteristics of the transistor due to irradiation with visible lightor ultraviolet light is small. Thus, the transistor has highreliability.

This embodiment can be freely combined with other embodiments.

Embodiment 9

In this embodiment, an electronic device and a lighting device to whicha light-emitting device of one embodiment of the present invention isapplied will be described with reference to FIGS. 18A to 18E.

A light-emitting device of one embodiment of the present invention is ahighly reliable light-emitting device having flexibility in which damageto a transistor and/or an organic EL element due to bending or curvingwhen physical power is externally applied can be suppressed.Accordingly, with the use of such a light-emitting device, a highlyreliable electronic device that has high resistance to bending, curving,and the like can be achieved.

As examples of the electronic devices, the following can be given:television devices (also referred to as televisions or televisionreceivers), monitors of computers or the like, cameras such as digitalcameras or digital video cameras, digital photo frames, mobile phones(also referred to as cellular phones or cellular phone devices),portable game machines, portable information terminals, audioreproducing devices, large game machines such as pachinko machines, andthe like.

The light-emitting device of one embodiment of the present invention canbe incorporated along a curved inside/outside wall surface of a house ora building or a curved interior/exterior surface of a car.

FIG. 18A illustrates an example of a mobile phone. The mobile phone 7400is provided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using a light-emitting device for the display portion7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 18A is touched with a finger or the like, data can be input intothe mobile phone 7400. Further, operations such as making a call andinputting a letter can be performed by touch on the display portion 7402with a finger or the like.

With the operation buttons 7403, power ON or OFF can be switched. Inaddition, a variety of images displayed on the display portion 7402 canbe switched; switching a mail creation screen to a main menu screen, forexample.

The display portion 7402 includes a light-emitting device of oneembodiment of the present invention; thus, a reliable mobile phonehaving a curved display portion can be provided.

FIG. 18B is an example of a wristband-type portable display device. Aportable display device 7100 includes a housing 7101, a display portion7102, an operation button 7103, and a sending and receiving device 7104.

The portable display device 7100 can receive a video signal with thesending and receiving device 7104 and can display the received video onthe display portion 7102. In addition, with the sending and receivingdevice 7104, the portable display device 7100 can send an audio signalto another receiving device.

With the operation button 7103, power ON/OFF, switching displayedvideos, adjusting volume, and the like can be performed.

The display portion 7102 includes a light-emitting device of oneembodiment of the present invention; thus, a reliable portable displaydevice having a curved display portion can be provided.

FIGS. 18C to 18E illustrate examples of a lighting device. Lightingdevices 7200, 7210, and 7220 each include a stage 7201 provided with anoperation switch 7203 and a light-emitting portion supported by thestage 7201.

The lighting device 7200 illustrated in FIG. 18C includes alight-emitting portion 7202 having a wave-shaped light-emitting surface,which is good-design lighting device.

A light-emitting portion 7212 included in the lighting device 7210illustrated in FIG. 18D has two convex-curved light-emitting portionssymmetrically placed. Thus, all directions can be illuminated with thelighting device 7210 as a center.

The lighting device 7220 illustrated in FIG. 18E includes aconcave-curved light-emitting portion 7222. This is suitable forilluminating a specific range because light emitted from thelight-emitting portion 7222 is collected to the front of the lightingdevice 7220.

The light-emitting portion included in each of the lighting devices7200, 7210, and 7220 are flexible; thus, the light-emitting portion maybe fixed on a plastic member, a movable frame, or the like so that anemission surface of the light-emitting portion can be bent freelydepending on the intended use.

Note that although the lighting device in which the light-emittingportion is supported by the stage is described as an example here, ahousing provided with a light-emitting portion can be fixed on a ceilingor suspended from a ceiling. Since the light-emitting surface can becurved, the light-emitting surface is curved to have a depressed shape,whereby a particular region can be brightly illuminated, or thelight-emitting surface is curved to have a projecting shape, whereby awhole room can be brightly illuminated.

Each light-emitting portion includes a light-emitting device of oneembodiment of the present invention; thus, a reliable lighting devicehaving a curved light-emitting portion can be provided.

FIG. 19A illustrates an example of a portable display device. A displaydevice 7300 includes a housing 7301, a display portion 7302, operationbuttons 7303, a display portion pull 7304, and a control portion 7305.

The display device 7300 includes a rolled flexible display portion 7102in the cylindrical housing 7301.

The display device 7300 can receive a video signal with the controlportion 7305 and can display the received video on the display portion7302. In addition, a battery is included in the control portion 7305.Moreover, a terminal portion for connecting a connector may be includedin the control portion 7305 so that a video signal or power can bedirectly supplied from the outside with a wiring.

By pressing the operation buttons 7303, power ON/OFF, switching ofdisplayed videos, and the like can be performed.

FIG. 19B illustrates a display device 7300 in a state where the displayportion 7302 is pulled out with the display portion pull 7304. Videoscan be displayed on the display portion 7302 in this state. Further, theoperation buttons 7303 on the surface of the housing 7301 allowone-handed operation. The operation button 7303 is provided not in thecenter of the housing 7301 but on one side of the housing 7301 asillustrated in FIG. 19A, which makes one-handed operation easy.

Note that a reinforcement frame may be provided for a side portion ofthe display portion 7302 so that the display portion 7302 has a flatdisplay surface when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

The display portion 7302 includes the light-emitting device of oneembodiment of the present invention. Thus, the display portion 7302 is adisplay device which is flexible and highly reliable, which makes thedisplay device 7300 lightweight and highly reliable.

Needless to say, there is no particular limitation to theabove-described electronic device or the above-described lighting deviceas long as the light-emitting device of one embodiment of the presentinvention is included.

This embodiment can be freely combined with other embodiments.

Example

A light-emitting device of one embodiment of the present invention wasmanufactured, and the characteristics of a transistor included in thelight-emitting device were evaluated. In this example, the results ofthe evaluation will be described.

In this example, Sample a that is a light-emitting device of oneembodiment of the present invention and Comparative sample b that is alight-emitting device of a comparative example were manufactured. Samplea includes, between the first flexible substrate 101 and the secondflexible substrate 111, the planarization layer 105, the transistor 103provided on the one surface side of the planarization layer 105, theorganic EL element 107 provided on the other surface side of theplanarization layer 105, and the adhesive layer 109 for bonding the pairof substrates (see FIG. 1). Comparative sample b includes aplanarization layer, a transistor, and an organic EL element, which aresimilar to those in Sample a, in a space surrounded by a pair of glasssubstrates (without flexibility) and an adhesive layer. The space was inan inert atmosphere.

First, a process for manufacturing Sample a will be described. In thisexample, the steps are described with reference to FIGS. 6A to 6E.

First, a 200-nm-thick silicon oxynitride film was formed as a base filmover a glass substrate serving as the formation substrate 501. Then,washing was performed using a hydrogen fluoride aqueous solution of0.5%. This step leads to improvement in adhesion between the base filmand the separation layer 503 to be formed later.

Next, a 30-nm-thick tungsten film was formed over the base film as theseparation layer 503, and the layer 505 was formed over the separationlayer 503 (FIG. 6A). In this example, the layer 505 includes aninsulating film, the transistor 103, the planarization layer 105, theorganic EL element 107, and the like.

As the layer 505, first, an insulating film was formed over theseparation layer 503. For the insulating film, a 600-nm-thick siliconoxynitride film, a 200-nm-thick silicon nitride film, a 200-nm-thicksilicon oxynitride film, a 140-nm-thick silicon nitride oxide film, anda 100-nm-thick silicon oxynitride film were stacked in this order. Afterthat, heat treatment was performed at 480° C. in a nitrogen atmospherefor one hour. Then, washing was performed using a hydrogen fluorideaqueous solution of 0.5%.

Next, the transistor 103 was formed over the insulating film. A200-nm-thick tungsten film was formed by a sputtering method over theinsulating film. A mask was formed over the tungsten film by aphotolithography process, and part of the tungsten film was etched withthe use of the mask, so that the gate electrode was formed.

Next, a gate insulating film was formed over the gate electrode. For thegate insulating film, a 90-nm-thick silicon nitride film and a50-nm-thick silicon oxynitride film were stacked in this order.

Next, an oxide semiconductor film was formed to overlap with the gateelectrode with the gate insulating film provided therebetween. Here, a35-nm-thick oxide semiconductor film was formed over the gate insulatingfilm by a sputtering method, a mask was formed over the oxidesemiconductor film by a photolithography process, and part of the oxidesemiconductor film was etched with the use of the mask, whereby theoxide semiconductor film was formed.

The oxide semiconductor film was formed in such a manner that asputtering target where In:Ga:Zn=1:1:1 (atomic ratio) was used, argonwith a flow rate of 50 sccm and oxygen with a flow rate of 50 sccm weresupplied as a sputtering gas into a reaction chamber of a sputteringapparatus, the pressure in the reaction chamber was adjusted to 0.7 Pa,and a direct-current power of 5 kW was supplied. Note that the oxidesemiconductor film was formed at a substrate temperature of 170° C.

After that, heat treatment at 450° C. in a nitrogen atmosphere for onehour and then heat treatment at 450° C. in an oxygen atmosphere for onehour were performed.

Next, part of the gate insulating film was etched to expose the gateelectrode, and then a pair of electrodes (a source electrode and a drainelectrode) in contact with the oxide semiconductor film was formed.Here, a conductive film was formed over the gate insulating film and theoxide semiconductor film. For the conductive film, a 50-nm-thicktungsten film, a 400-nm-thick aluminum film, and a 100-nm-thick titaniumfilm were stacked in this order. Then, a mask was formed over theconductive film by a photolithography process, and part of theconductive film was etched with the use of the mask, whereby the pair ofelectrodes was formed.

Then, the oxide semiconductor film was exposed to oxygen plasma whichwas generated in such a manner that an upper electrode provided in thereaction chamber was supplied with high-frequency power of 150 W withthe use of a 27.12 MHz high-frequency power source.

Next, a protection film was formed over the oxide semiconductor film andthe pair of electrodes. Here, for the protection film, a first oxideinsulating film, a second oxide insulating film, and a nitrideinsulating film were formed.

First, a 50-nm-thick silicon oxynitride film was formed as the firstoxide insulating film and a 400-nm-thick silicon oxynitride film wasformed as the second oxide insulating film.

Next, heat treatment was performed to remove water, nitrogen, hydrogen,and the like from the first oxide insulating film and the second oxideinsulating film. Here, the heat treatment was performed in an atmosphereof nitrogen and oxygen at 350° C. for one hour.

Next, the substrate was transferred to a treatment chamber under reducedpressure and heated at 350° C., and a nitride insulating film was formedover the second oxide insulating film. Here, as the nitride insulatingfilm, a 100-nm-thick silicon nitride film was formed.

Then, part of the protection film was etched to form an opening fromwhich part of the pair of electrodes was exposed.

The planarization layer 105 was formed over the protection film. Here,the protective film was coated with a composition, and exposure anddevelopment were performed, so that the planarization film 150 having anopening through which the pair of electrodes is partly exposed wasformed. Note that as the planarization layer 105, a 1.5-μm-thick acrylicresin layer was formed. After that, heat treatment was performed. Theheat treatment was performed in a nitrogen atmosphere at 250° C. for onehour.

Next, a conductive film electrically connected to part of the pair ofelectrodes was formed. Here, for the lower electrode of the organic ELelement 107, a 50-nm-thick titanium film, a 200-nm-thick aluminum film,and a 3-nm-thick titanium film were formed by a sputtering method, andfurthermore, an indium tin oxide containing silicon oxide (ITSO) filmwas formed as an optical adjustment layer. The thickness of the ITSOfilms of light-emitting elements in a red pixel, a green pixel, and ablue pixel were 82 nm, 45 nm, and 5 nm, respectively.

Then, a partition wall was formed to cover an end portion of theconductive film. Here, a 1.0-μm-thick polyimide resin film was formed asthe partition wall. After that, heat treatment was performed. The heattreatment was performed in an atmosphere containing nitrogen at 250° C.for one hour.

A spacer with an inverse tapered shape was formed on the partition wall.Here, a 2.0-μm-thick spacer was formed with the use of a negativephotosensitive resin. After that, heat treatment was performed. The heattreatment was performed in an atmosphere containing nitrogen at 250° C.for one hour.

Next, an EL layer and an upper electrode were formed over the conductivefilm. All the light-emitting elements in different color pixels have thesame structures of the EL layer and the upper electrode. Thelight-emitting element of this example is a tandem light-emittingelement in which the EL layer includes a fluorescence-emitting unitincluding a blue light-emitting layer and a phosphorescence-emittingunit including a green light-emitting layer and a red light-emittinglayer. As the upper electrode, a 15-nm-thick film was formed byco-evaporation of magnesium and silver. Furthermore, a 70-nm-thick ITOfilm was formed over the upper electrode. Through the above steps, theorganic EL element 107 was formed.

A base film and the separation layer 553 were formed over the glasssubstrate serving as the formation substrate 551. The base film and theseparation layer 553 were formed in a manner similar to that of the basefilm and the separation layer 503 formed over the formation substrate501. Then, the layer 555 to be separated was formed over the separationlayer 553 (FIG. 6B). In this example, the layer 555 to be separatedincludes an insulating film, a color filter, and the like.

For the layer 555 to be separated, first, an insulating film was formedover the separation layer 553. The insulating film was formed in amanner similar to that of the insulating film included in the layer 505.A color filter was formed over the insulating film, and then a20-nm-thick ITO film was formed over the color filter. Here, the ITOfilm has higher wettability with respect to a material used for theadhesive layer 109 than the color filter; accordingly, generation of airbubbles can be prevented when the formation substrate 501 and theformation substrate 551 are bonded, whereby sealing of an element can befavorably performed.

Next, the formation substrate 501 and the formation substrate 551 werebonded with the adhesive layer 109 (FIG. 6C). A UV curable resin wasused for the adhesive layer 109.

Then, the layer 505 was separated from the formation substrate 501 alongthe separation layer 503. Next, the layer 505 exposed by the separationfrom the formation substrate 501 was bonded to the first flexiblesubstrate 101 with the use of the adhesive layer 123 (FIG. 6D).Similarly, the layer 555 was separated from the formation substrate 551along the separation layer 553. Next, with the adhesive layer 187, thesecond flexible substrate 111 was bonded to the layer 555 exposed by theseparation from the formation substrate 551 (FIG. 6E). A 20-μm-thickplastic film was used for each of the first flexible substrate 101 andthe second flexible substrate 111. The first flexible substrate 101 andthe second flexible substrate 111 are each a substrate with acoefficient of thermal expansion of 10 ppm/K or lower and less likely tobe deformed by heat. A UV curable resin was used for the adhesive layer123 and the adhesive layer 187.

Finally, the FPC 4505 was attached to each electrode of an input-outputterminal portion with the use of an anisotropic conductive member.

Through the above steps, Sample a of this example was manufactured.

On the other hand, in Comparative sample b, the transistor 103, theplanarization layer 105, and the organic EL element 107 were formeddirectly (without the separation layer) on the glass substrate servingas a supporting substrate. Then, a counter substrate and the supportingsubstrate were bonded with the use of an adhesive layer in areduced-pressure atmosphere, so that the transistor 103, theplanarization layer 105, the organic EL element 107, the color filter,and the like were sealed. A UV curable resin was used for the adhesivelayer. A glass substrate provided with a color filter was used as thecounter substrate. Finally, an FPC was attached to each electrode of aninput-output terminal portion with the use of an anisotropic conductivemember. Through the above steps, Comparative sample b of this examplewas manufactured.

FIG. 20 shows the Vg-Id characteristics of the transistor in Sample aand the transistor in Comparative sample b. A bold solid line representsthe result of Sample a and a fine solid line represents the results ofSample b. Here, measurements were performed by connecting an FPCterminal of each sample to a measuring system. The channel length andthe channel width of each of the measured transistors were L/W=3 nm and1000 nm. The thickness of the gate insulating film of each of themeasured transistors was 100 nm when converted into a silicon oxide film(with a dielectric constant of 4.1).

The results in FIG. 20 show that there is no large difference betweenthe characteristics of the transistor in Sample a and thecharacteristics of the transistor in Comparative sample b. Therefore, byapplication of one embodiment of the present invention, a flexiblelight-emitting device, which has reliability as high as a conventionalnon-flexible light-emitting device, can be manufactured.

This application is based on Japanese Patent Application serial no.2012-107283 filed with Japan Patent Office on May 9, 2012, JapanesePatent Application serial no. 2012-107284 filed with Japan Patent Officeon May 9, 2012, Japanese Patent Application serial no. 2012-108190 filedwith Japan Patent Office on May 10, 2012, and Japanese PatentApplication serial no. 2013-044857 filed with Japan Patent Office onMar. 7, 2013, the entire contents of which are hereby incorporated byreference.

1. (canceled)
 2. A light-emitting device comprising: a planarizationlayer; a transistor; a light-emitting element; and a neutral plane whichdoes not easily expand or contract.
 3. The light-emitting deviceaccording to claim 2, wherein the neutral plane is positioned in theplanarization layer, in the transistor, in the light-emitting element,or near both the transistor and the organic EL element.
 4. Thelight-emitting device according to claim 2, further comprising: anorganic resin layer over a first substrate; a terminal electrode overthe organic resin layer; the transistor over the organic resin layer;the planarization layer over the transistor; the light-emitting elementover the planarization layer, the light-emitting element electricallyconnected to the transistor; an adhesive layer over the planarizationlayer and the light-emitting element; a second substrate over theadhesive layer; and a third substrate.
 5. A light-emitting device,wherein a distance from a bottom surface of a first substrate to a topsurface of a planarization layer is 0.8 to 1.2 times as large as adistance from the top surface of the planarization layer to a topsurface of a second substrate.
 6. A light-emitting device comprising: afirst driving circuit; a second driving circuit; an FPC; and a pixelportion, the pixel portion comprising: a transistor over a firstflexible substrate; a planarization layer over the transistor; anorganic EL element, electrically connected to the transistor, over theplanarization layer; and a second flexible substrate over the organic ELelement, wherein the first driving circuit and the second drivingcircuit are provided over the first flexible substrate, wherein thefirst driving circuit is provided in a periphery of the pixel portion,and wherein the second driving circuit is provided at the opposite sideof the first driving circuit with the pixel portion providedtherebetween.
 7. The light-emitting device according to claim 6, whereina neutral plane is positioned in the planarization layer, in thetransistor, in the organic EL element, or near both the transistor andthe organic EL element.
 8. The light-emitting device according to claim6, wherein a thickness from the first flexible substrate to a topsurface of the planarization layer is 0.8 times or more and 1.2 times orless as large as a thickness from the second flexible substrate to thetop surface.
 9. The light-emitting device according to claim 6, whereinthe FPC comprises a region which is not sandwiched between the firstflexible substrate and the second flexible substrate.